bobthefish: added promt support for virtualfish

It's a promt for https://github.com/adambrenecki/virtualfish

![screenshot](http://i.imgur.com/lmndI0k.png)

`◰` means virtualenv (unfortunately there is no "python" or "snake" unicode symbols in common fonts), `³` means the version of python.

UPD: sadly I didn't know about this PR: https://github.com/bpinto/oh-my-fish/pull/144 I don't know why was it closed.
I don't use it very much too. But when I use it I need to know it exactly. Today I wasted 10 minutes to figure out why doesn't my python programs outside of virtualenv doesn't work.