add_logo: Add Marks
In purplezippo/ggpkt: YIK Themes and Customized Geoms for Data Vis.

Include logo, copyright statement and watermarks.

add_logo(ggobj, logo.size = 0.1, logo.loc = c(0.5, 0.5))

add_copyright(ggobj, just = c("center", "bottom"),
  text = "copyright: www.yikdata.com", col = "black")

add_watermark(ggobj, text = "www.yikdata.com", angle = 0,
  alpha = 0.4)

`ggobj`	A ggplot object.
`logo.size`	Controls the size of logo image.
`logo.loc`	Coordinates of the center point, which controls the position of logo image.
`just`	A string or numeric vector specifying the justification of the viewport relative to its (x, y) location. If there are two values, the first value specifies horizontal justification and the second value specifies vertical justification. Possible string values are: `"left"`, `"right"`, `"centre"`, `"center"`, `"bottom"`, and `"top"`. For numeric values, 0 means left alignment and 1 means right alignment.
`text`	Character to add to the figure.
`col`	Colour for text.
`angle`	The angle of text.
`alpha`	Alpha channel for transparency