Andrewburnell/release changelog

Added in an explicit changelog confirmation for each 'sbt release' to help prevent accidental release of other contributors' code and give a releaser a more sane digest.  This generates a list of all commits since the last release with some metadata and a confirmation prompt.
Maybe match on "v[0-9]*" to make it a bit less likely that some other tag starting with "v" could get in the way?