Add an option to only allow incremental package versions to be published

[Tohaker]

It appears that using the semver.diff function only checks if the package version is different, and subsequently ignores whether the version is actually a higher or lower version.

I'd like to add an option that uses semver.compare to only run if the new package version is higher than that already published. I'm thinking something along the lines of greater-version (default: false, for backwards compatibility).

I'll raise a PR with an implementation too.

[mcous]

Resolved by #44 and #58. Not yet released to a tag, but in the mean time should be available via the commit directly: 8a7309043b3e67e38892ed629a136899c3cf166a