Transparency and tint support.

New configuration option 'alpha' which adjusts alpha transparency
(0 is transparent, 255 is opaque).

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Merge branch 'transparency' of https://github.com/ezyang/xmobar

Conflicts:
	src/Config.hs
	src/Main.hs
	src/Parsers.hs
	src/XUtil.hsc
	src/Xmobar.hs
Default to opaque
Documentation nits
Fix: use the right background area in transparency
Linting and defaults
Credits for Edward Z Yang