Fix issue related to #7615, #7747 and #6346
After the update, aws prompt (which should be visible out of the box) disappears when a user uses a theme af-magic, because of fact that plugins are loaded before themes.
This pull request fixes issue with not showing aws prompt in theme af-magic, by appending RPROMPT in theme af-magic instead overwriting.
- Add mercurial support to af-magic, so now the vcs prompt will show up in either a git or hg repository
- The virtualenv prompt was white and bumped up against the user@hostname output
- Fixed that so its green (which I thought highlighted it more thematically) and has a space before user@hostname
