Add pre-release support

Hi @deanishe,
sometimes I release beta versions of my workflows.
I'd also like to tag them in GitHub just to keep track of everything.

GitHub releases can be flagged as pre-release.
If we simply ignore those in the updater, I could tag beta versions as pre-releases.

What do you think?
Merge pull request #39 from fniephaus/patch-1

Add pre-release support