tidyx: modified tidyverse functions for interractive use

takes the features of summarize_if and summarize_at, in case we want to combine both, and supports a formula or function for the .at parameter, which will be applied on names. Also supports grouping through .by.

1 2	summarize_ifx(.tbl, .funs, ..., .at = names(.tbl), .if = ~TRUE, .by = NULL)

`.tbl`	A `tbl` object.
`.funs`	List of function calls generated by `funs()`, or a character vector of function names, or simply a function. Bare formulas are passed to `rlang::as_function()` to create purrr-style lambda functions. Note that these lambda prevent hybrid evaluation from happening and it is thus more efficient to supply functions like `mean()` directly rather than in a lambda-formula.
`...`	Additional arguments for the function calls in `.funs`. These are evaluated only once, with tidy dots support.
`.at`	equivalent of .vars in mutate_at, but enhanced
`.if`	equivalent of .predicate in mutate_if
`.by`	a vector or list of variables as characters or given by `vars`