Provide version control info in shell prompts

Current versions

vcprompt requires the following formula to be installed:
sqlite 3.23.1 Command-line interface for SQLite

Recent formula history

Mike McQuaid Use hash rockets again. (#5177)
Mike McQuaid Use Ruby 1.9+ symbol hash keys in all formulae. (#4942)
Nikolaus Wittenstein Add descriptions to all remaining homebrew packages
Jack Nagel Prefer :using over made-up URL schemes
Jack Nagel vcprompt: modernize autotools deps

Formula code at GitHub