use haml when available

Note: there's probably a much better way to do this.

I just looked at how the haml-rails gem was doing it
and played with pry a little.  It seems to work too :)
make shell commands valid
only use supported templates
add support for slim templates
Merge pull request #44 from joshuarh/use-haml-when-available

use haml when available
Merge pull request #45 from joshuarh/valid-shell-commands

make shell commands valid