Use strings instead of numbers for PR types.

The short strings are easier to remember and type than using a
zero-based array.
Store the pull requests by type code rather than type string.

The type code is more programmatic in nature and looks better when
basing logic off of it.
Add the ability to ignore a pull request.  Fixes #22.

Some pull requests don't deserve to be in the release notes.  Rather
than force the user to edit them out after-the-fact, allow them to
ignore the unnecessary pull requests on entry.
Merge pull request #25 from nubs/named-pull-request-options

Add the ability to ignore a pull request.