Fix segfaults for :toggle commands without arguments

Fixes #259
Closes #260
Add compatibility mappings for obsolete color names

This replaces old color mappings for obsolete color names with new ones
introduced when color settings were moved to tigrc. While we are at it
also update tigrc(5) to reflect current color names. Finally, remove
useless main-revgraph color name which allows to color space between
revgraph and the commit text.