gibble: Geometry Decomposition

Thank you to @jennybc for prompting these changes:

Before attempting to read/write form the clipboard, clipr_available() will first explicitly check if it is being run non-interactively, and if so, if the CLIPR_ALLOW environment variable has been set. This will hopefully prevent starting spurious Linux processes during CRAN tests.
Out of an abundance of caution, read_clip() now does the same interactive/envvar check that write_clip() does.
Some documentation clarifications

Thank you to @wangyuchen for making the following suggestions:

To make clipr more pipe-friendly, write_clip() now defaults to return_new = FALSE, and will instead return the initial object that was passed in. To get the old behavior, pass return_new = TRUE
In an effort to make write_clip() and read_clip_tbl() more symmetrical, write_clip() now defaults to writing out row and column names when they exist.
Introduces write_last_clip(), a wrapper function for write_clip(.Last.value)

To comply with CRAN policy, write_clip() will now error by default if run in a non-interactive session. Non-interactive use must be explicitly enabled by setting an environment variable CLIPR_ALLOW=TRUE.
Documented that the default behavior when writing matrices to write_clip() is col.names = FALSE

Correct a formatting error by adding and separation character to tables when they are being written with rownames.

Introduces dr_clipr(), which gives informative suggestions for software and configuration requirements when accessing the clipboard on X11-based systems.

Due to poor testing and configuration options, clipr was not delivering on its promised support for xsel :( This has now been fixed, with more complete Travis tests, and some core fixes by @milesmcbain.

Suppress an erroneous warning on OS X / X11 systems when trying to write an empty string to the clipboard.
Fix error when NA is passed to write_clip(). This will now write "NA" to the clipboard.
Fix error when passing NULL or an empty vector (e.g. character(0)). This will now write "" to the clipboard.

Fixes a breaking bug that caused clipr_available to erroneously return FALSE. Thank you to @krivit for catching this.
Introduces better testing of clipr_available to properly evaluate it on Travis CI.

Introduces clipr_available which checks to see if the system clipboard is writeable/readable. This may be useful if you are developing a package that relies on clipr and need to ensure that it will skip tests on machines (e.g. CRAN, Travis) where the system clipboard may not be available. Thank you to @jennybc for this suggestion.
Implements genuine testing of clipr functionality with thanks to some deft environment variable settings added by @jennybc.
Two Rstudio addins: one to copy the value returned when a highlighted expression is evaluated, and another that copies the console output.

## clipr 0.2.1

Introduces read_clip_tbl, a convenience function that takes tab-delimited text from read_clip (such as that copied from a spreadsheet) and parses it with read.table. Thank you to Steve Simpson (@data-steve) for the original PR.
write_clip(object_type = "table") has a new internal implementation (writing to a temporary file rather than using capture.output) which should dramatically shorten the time it takes to write very large tables to the clipboard. Thank you to @r2evans for this suggestion.

## clipr 0.2.0

Several changes to write_clip - The separator to be used when writing a character vector can now be explicitly declared using breaks. breaks=NULL will default to system-specific line breaks for both vectors and tables. - write_clip will default to formatting data.frames and matrices with write.table, allowing easy pasting of tabular objects into programs like Excel. Option object_type="auto" will check the object type to decide on the correct formatting, or the user may explicitly state object_type="table" or object_type="character". - clipr will default to sane system-specific options for write.table(), however you may pass any custom desired options via write_clip - return_new=TRUE (the default behavior) will return the formatted character string that was passed to the system clipboard, while write_clip(return_new=FALSE) will return the original object.
Introduces clear_clip, a wrapper function for write_clip("") for easy clearing of the system clipboard.