Allow the user to specify the target branch to merge onto. Fixes #32.

master is common, but definitely not the only branch people use.
Especially with support for multiple version lines (develop, v1.x, v2.x,
etc.).
Merge pull request #36 from nubs/target-branch

Allow the user to specify the target branch to merge onto. Fixes #32.
Allow the user to specify what the previous tag was. Fixes #33.

You can't assume that the user is releasing a tag for the same branch as
the latest tag.  Previous versions that are still supported can also
need to be tagged.