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.
`--require spec_helper` is more clear than `-rspec_helper`
Address feedback.

- Add missing back ticks.
- Combine `filter_run`/`run_all_when_everything_filtered`
  configs and explanations.
- Remove the `expose_dsl_globally = false` config for now,
  with the plan to potentially add it back in a later
  release if user feedback about it is positive.
- Re-order things a bit.
Merge pull request #1219 from rspec/rspec-init

Improve `rspec --init`.