Add mercurial support for bobthefish

This is @exdis' PR #253, plus a rebase so that it works on top of the recent changes to bobthefish.

In addition, it fixes a few issues with that PR and adds:

 * Use `hg root` instead of `hg stat` to check whether we're inside an hg project… this seems to be consistently about 20% faster to check.
 * Fix theme when git or hg is not present
 * Add a mercury glyph to the front of the SCM segment to distinguish between being inside a git and a mercury project.
 * Remove a bit of duplication, standardize some func names, and stop saving $status to $RETVAL.