Add support for pre-releases

[nolar]

Currently, all pre-releases are filtered out. The one that is not marked as a pre-release (v1.18.2-rc3+k3s1) is sorted improperly — it goes as if it is newer that its public release (v1.18.2+k3s1).

A/C:

• Version sorting is fixed so that RCs go after their public releases.
• RCs can be used by mentioning their name explicitly without additional options.
• RCs can be automatically detected by specifying with: prereleases: true and partial versions (e.g. v1.18).