Support --from-path for quicken CLI

[chrahunt]

Currently it is not straightforward to do anything with quicken but quickly test a single file. --from-path would search PATH for the applicable script and run it, so an introduction could look like alias qpip="quicken run --from-path pip --". This would probably be more approachable than the less common --module suggested in #56.

[chrahunt]

A basic test on my machine shows that a naive import quicken._internal.cli.cli followed by shutil.which(sys.argv[1]) takes 40-70ms. If we can get this done around that time it might be OK, otherwise we may want to take a different approach.