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.
Don't see a reason for this condition, just export it in the environ. Oh, please don't use backslashes and instead Python's implied line continuation in braces.