Consider deprecating multiple step modules

[hyperthunk]

The use of multiple step modules for a single feature (file) seems a little haphazard. It's intent appears to be sharing of code (i.e., step function definitions) between modules, but I think there are other, cleaner ways to do this.

The use of helper modules, for example, would alleviate the need for this, and is supported on the cli (see https://github.com/hyperthunk/cucumberl/blob/master/src/cucumberl_cli.erl#L45) and could easily be moved into the main cucumber module.

Also, not processing multiple step modules would simplify error handling and negate the requirement to always implement a stub function clause in your modules, i.e.,

%% no more of these!!!
step(_, _) -> undefined.

Please let me know if you would consider this change, as I'd be happy to contribute.