Enhance VCS information

[eproxus]

• [ ] Remote branch ahead/behind (see http://eseth.org/2010/git-in-zsh.html)
• [ ] Stash count (see http://eseth.org/2010/git-in-zsh.html)

[eproxus]

More resources:

• https://github.com/zsh-users/zsh/blob/master/Misc/vcs_info-examples
• http://eseth.org/2010/hg-in-zsh.html
• https://github.com/Valodim/zsh-prompt-powerline/blob/master/hooks/vcs_info-githooks.zsh
• http://zsh.sourceforge.net/Doc/Release/User-Contributions.html#Quickstart