Add tracking mode to follow semantically-versioned releases

[jonathansick]

We have tracking modes for a branch and for various EUPS tags, but as more LSST packages adopt semantic versioning, we should add a tracking mode for semantic versions.

• semver: track latest semantic version (excluding rc, alpha, beta, dev releases)
• vsemver: track latest semantic version as above, but given a v prefix.

It would also be great to have a tracking mode for the latest patch of a given patch version. So that e.g. /v/3.4/ would track a 3.4.2 release if that was the latest patch of the 3.4.x series.