Optionally derive the step format from method name

[jeremydmiller]

Shamelessly rip off SpecFlow w/ syntax like this:

[Given]
public void Given_I_have_entered_NUMBER_into_the_calculator(int number)
{
  ...
}

but just this in Storyteller:

public void Given_I_have_entered_NUMBER_into_the_calculator(int number)
{
  ...
}