Skip bash-it type enable if already ran.

If there are files in the enable directory for a bash-it type, it will
skip asking if they want to enable any of that type.
OS specific dotfiles update

Move OS dotfiles to an os directory. This will ignore those *.symlink
files because they are more than 2 directories deep. Moved the
install_os_dotfiles from script/bootstrap to os/install.sh.