The doubletime theme improves the SCM status prompt for git.
However, it accomplishes this at the expense of other SCM
systems. This delegates prompt to the default bash-it prompt
function when the SCM is not git.
This change allows someone to change their host color by adding the
following to their `.bash_profile`:
# change to red
THEME_PROMPT_HOST_COLOR="red"
# change to green
THEME_PROMPT_HOST_COLOR="green"
Any variable inside the `colors.theme.bash` can be used and it will be
evaluated at the last minute.
This switches the doubletime theme over to using a variable for
determining what to display in the prompt. The original version's use
of a hard-coded `@local` means you would have that suffix displayed even
on remote machines.
