Move preview path default

This builds on @takashi's work, see #1185, with `ActionMailer` previews. It moves the setting of the default preview path into a Railtie, which mirrors how Rails configures it. This also fixes the broken generator due to a missing require. This was missed previously due to the Rails generators not causing the build to fail if they fail. A shim has been added to the example app generation to catch this in the future.

The test for the default preview path is clunky. We do not have a method of creating a lightweight Rails app to use directly from the specs. Instead this adds a custom lightweight script which is shelled out to and run, when the example app runs.
Are you intending to test this elsewhere?