Add virtualenv support to bobthefish powerbar theme

If VIRTUAL_ENV envvar is set, then prefix the basename in blue to the start of the powerbar
Also set the VIRTUAL_ENV_DISABLE_PROMPT envvar to stop virtualenv from adding its own prefix

Currently an issue when entering a virtualenv, because the fish script to enter a virtualenv returns non-zero exit status when VIRTUAL_ENV_DISABLE_PROMPT is set, but this is a minor issue that can be solved by virtualenv patch.

![capture](https://cloud.githubusercontent.com/assets/704526/2764173/e4d75ae8-ca0f-11e3-9173-727925e4339a.PNG)