Possibility to specify which shell to use

I use bashtest with great success on my zsh scripts by manually changing "bash" to "zsh" in bashtest.py:

\-    command = 'bash -c %s' % (quote(command))
+    command = 'zsh -c %s' % (quote(command))

This is a hack of course, and it would be great if this could be set on the command line or in a preference.
