[bobthefish] all the fish

Tons of bobthefish updates: 

 * Update virtualenv segment colors to match theme
 * Add a segment delimiter between virtualenv glyph and name
 * Speed up Git and hg segments (only find project root once, rather than 2-3 times, reuse this to know whether we're in hg / git)
 * Much faster hg project root detection (thanks @rominf!)
 * Add support for RVM and rbenv segment
 * Speed up and clean up git ahead/behind checking
 * Add config options for hiding certain prompt segments (thanks @rominf!)
 * Clean up multiple styles of stderr redirect, multiple styles of test syntax, etc

This includes all current bobthefish PRs, all mashed together, rebased and merge-conflict-resolved :)

Closes #256 
Closes #257 
Closes #260 
Closes #261