Add option to print matching checklist items during gap analysis

[johndgiese]

Currently, rdm gap only prints out the missing checklist items. It would be nice to provide an option that prints out the matching checklist items too. A few requirements of this option include:

• showing the filename and line number of all matches
• showing some context around the matches
• showing the checklist item description from the checklist files

This issue requires two reviews. First, a review of the design specification (including the example output format). Second, a review of the code construction and tests.