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.