Add a single insertion function covering all colors and attributes

[aaron-em]

@benma suggests in #4 that the library provide a single function which inserts (and provides as completion candidates) all available color and attribute codes:

Right now, normally I only want a color and/or an attribute, so splitting the color in two steps, fg/bg, is a bit distracting. Same for choosing between the two commands. If I have completion, completing for red and red (bg) and underline all works well, I think.

Should be pretty straightforward, given that from 1.3 we can now insert both foreground-only and background-only color codes, and adjacent color codes can be easily merged upon insertion in any case.

Tasks:

• Define rcirc-styles-insert-style as above
  • Completion candidates - alist of '(candidate . (insert-fun insert-args))?
• Add it to rcirc-styles-map