Improve `rspec --init`.

- Add lots more suggested settings to spec_helper.rb
  to give a great initial experience.
- Add a cucumber scenario that demonstrates that the
  settings generated by `rspec --init` are all valid.
  It doesn't demonstrate that they do what they are
  intended to do, but it protects against misspelled
  settings and whatnot.
- Remove old cruft.
- Put the init contents in their own files. Much
  cleaner than using heredocs.

I'd like some :eyes: on this to see what others think about all the settings this adds.  Is this overkill?  I think it'll make for a better starting experience to have more in the generated `spec_helper.rb` file.  Are there any settings I've added that shouldn't be included?  Anything I'm missing?

It would also be good to find a way to allow rspec-rails' generator to leverage this and build on top of it for the additional rails-specific stuff it adds.  I'm not sure of the best way to do that, though.

/cc @alindeman @JonRowe @xaviershay @soulcutter @samphippen