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 know this is closed, but I've only just seen it now that someone's linked to it on the virtualfish bugtracker. On my system, it's the other way around - `python2` links to `python`.
```
→ ls -l $VIRTUAL_ENV/bin
...
-rwxr-xr-x  1 adam  staff  12752 22 Nov 13:20 python
lrwxr-xr-x  1 adam  staff      6 22 Nov 13:20 python2 -> python
lrwxr-xr-x  1 adam  staff      6 22 Nov 13:20 python2.7 -> python
```
It might be a better idea to do `test -f $VIRTUAL_ENV/bin/python2` etc; I don't think virtualenv any guarantee that things will be symlinked in any particular way, but I'm fairly confident you're only ever going to have `python2` or `python3` in a single virtualenv's `bin/` directory.