New config options to throw errors for common, confusing scenarios

This adds three new config options to throw errors or skip caching certain commonly confusing scenarios.

These changes are being used with `better_cache_creation` in the repo that consumes this gem.

In this branch, the config settings will do the following:
 * Not save new cache files for any non-200 series responses (avoid confusing new cache files)
 * Raise an error when non-200s are received (so you know what to fix)
 * Allow new end-points to be hit (I'd like to flip this default once people are used to it and know what to do)

By default, puffing-billy's behavior doesn't functionally change -- there's just a new logging line.

Note: The tests for the non-successful settings still need work.  The existing test server has a hard-coded response code, [as seen here](https://github.com/centro/puffing-billy/blob/success_check/spec/support/test_server.rb#L28).