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.