Generate the 'next' version for a changelog with an Unreleased section

[baronfel]

I want to add a concept of 'the next prerelease version if there's an unreleased section', so that folks get a sane auto-generated version number when they do local builds. Logic would be something like:

• take the unreleased segment
• take the last release entry
• check the changes in the unreleased segment to determine what kind of bump (similar to what I created in FsAutoComplete)
• do that bump, but with a prerelease segment of -alpha.<COUNTER>, where counter is determined by the number of lines of changes in the unreleased portion overall. idea being that each PR or commit would map to a single change line in the changelog, so you'd have a minimal amount of traceability out of the box.

[baronfel]

Would also need a 'don't do that please' flag that folks could set in case they really didn't want the automatic version selection.

Big open issue is how to determine the expected bump, but I can just put a flag in the ground here and get feedback later.