Optionally modify prompt to reflect currently used profile.

[reillysiemens]

We could modify $PS1 or the like in the same fashion as virtualenv. This might involve some eval or source trickery. We might also investigate doing it in the style of git's __git_ps1.

An example prompt might look something like:
(default) user@localhost ~$
or
(work) user@fancy-company ~/secret/files$

[RadicalZephyr]

Two notes on implementation:

• we want this to be opt-in, so it should probably be configurable by environment variables.
• this should work across multiple shells, even though bash uses $PS1 and zsh uses $PROMPT etc.

A good source of inspiration are the __git_ps1 command and how it is configured, and rbenv init command.