Add support for `which` command as per request #36.

[shaleh]

As the subject says, add support for multirust which cmd as per #36.

[brson]

Thanks! Looks good except for the notes.

[brson]

Thanks again.

[tamird]

@brson perhaps the error handling is needed after all; running multirust which without an argument results in a puzzling error message /usr/local/bin/multirust: line 1288: $1: unbound variable

[brson]

@tamird oh, hm, thanks for verifying. I expected the input to be sanitized elsewhere, probably during argument parsing, and thought that check was defensive. Do you mind putting that check in handle_command_line_args? There are some existing argument checks there to crib off of.