new feature: optional prompt for envshell

this sets a new env variable called ``ENVDIR_PS1``
before running a new shell so for example in bash
it can be used to set a new shell prompt
(similar to virtualenv)

all you'd need to do is add a line similar to this
at the bottom of your .bashrc

    PS1=${ENVDIR_PS1:-${PS1}}

so after running ``envshell ./test/`` you would get ``[envdir:test]``
at the beginning of your prompt.
OK. I will avoid backslashes then. As for the condition I wanted to prepend the envshell directory name to ``PS1`` and store it in a different variable since the default ``~/.bashrc`` overrides ``PS1``. I admit that this was focused to much on bash and Daniel already pointed that out. Will try to use a more generic approach.