Add virtualenv support to the bobthefish 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.