Add command to list all available step definitions

This should solve #46
I found basically to approach, the first one was to manually scan the files, and the second one (the one that I used for this PR) was to use cucumber internals to get the steps definitions.

I've also added hirb to have nice output (sample output https://gist.github.com/sarcilav/5267167)

PD: As this is my first PR that plays a bit with the internals of cucumber and gauntlt, I'm not sure if I'm missing important things within code style and contribution rules.