Add mtahmed theme.

--HG--
extra : amend_source : e3eb545150f7c4aa5ccaca80a64649419d154e6d

- existence of dir
- and also if it is already in the path list

It's the gianu theme from oh-my-zsh but converted for oh-my-fish

Fix `set $PATH` mistake.

This also fixes issues for Homebrew users if the followed the caveat to
set PYENV_ROOT to use the homebrew created directories.

rbenv: Detect $RBENV_ROOT and act accordingly.

Same fix as with pyenv, if RBENV_ROOT is set, use that.

This also fixes issues for Homebrew users if they followed the caveat to
set RBENV_ROOT to use the homebrew created directories.

PHP HTTP server