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.
Make release name nicer when it is left blank.  Fixes #26.
Make final question easier to understand.

The wording of the question was confusing.  I believe a simple
"Continue?" will be easier to follow.
Merge pull request #27 from nubs/empty-release-name

Make release name nicer when it is left blank.  Fixes #26.
Allow the user to amend the release notes before submission. Fixes #13.

Often enough, the release notes generated need a little modification.
For example, perhaps they don't have enough information in just the pull
request title to make informative release notes of.

This will now pop up the user's preferred editor to allow them to edit
the release notes in a freeform manner.
Merge pull request #31 from nubs/customize-release-notes

Allow the user to amend the release notes before submission. Fixes #13.