Support for multiple releases (#8)

The approval test here https://github.com/JakeGinnivan/GitReleaseNotes/blob/b0d51cfbab4e0d713fe28eafec34145a40e6141c/src/GitReleaseNotes.Tests/ReleaseNotesWriterTests.MultipleReleases.approved.txt shows the support off.

Fixes issue #8