Add `fold` option to avoid version code jumps such as v1.2.3 -> v8.6.3

[FelixZY]

I've used semantic-release before where each version increment is quite minimal.

E.g.

fix: lorem ipsum
feat: lorem ipsum
fix: lorem ipsum

would result in a version bump to major.minor+1.0 rather than major.minor+1.patch+1, which is the behavior of this repo. This issue was discussed in #11 where @xotahal welcomed contributions - so here it is :wink: .

#11 also discusses support for "locked" version numbers. However, this is not something I have implemented at this time as I think it warrants further discussion first. Personally, I think the behavior of semantic-release should be imitated.