Add a link, so the warning will now link to rails-bestpractices.com

This link provides a description and a solution to the problem.
Call RailsBestPractices directly to get access to the array of errors.

We now have the URL, so we can provide a link to rails-best-pracitces.com
By calling the lib directly we can avoid IO overhead, however we want to
be sure that the RailsBestPractices maintains the interface we are
expecting, since it's not a publicly advertised interface.
I have added some specs that will error if the interface to
RailsBestPractices changes.

Change the output format to Yaml, so we can parse the URL.

Changing the output format for RailsBestPractices allows us to parse the
url which provides a link to a page on rails-bestpractices.com.  This page
explains the warning and provides a solution.

Add helper methods for `command` and `output_file`, so then can easily
be stubbed in test.

Update test to work with Yaml output form RailsBestPractices.

Add a test yml file that mimics output from running
RailBestPractices with the yaml format option.  Stubbed the
`output_file` method to return the yaml file during test.
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