Prompt formatter API

Modules that provides FORMATTER_DICT.

• Base prompt formatter (xonsh.prompt.base)
• CWD related prompt formatter (xonsh.prompt.cwd)
• Virtualenv prompt formatter (xonsh.prompt.env)
• Informative git status prompt formatter (xonsh.prompt.gitstatus)
• Jobs prompt formatter (xonsh.prompt.job)
• Version control branch info prompt formatter (xonsh.prompt.vc)