[Feature Request] Customize commit message by semver level of dependency update

[zypA13510]

Background

I'm writing an adapter for an upstream package. And in most situations, an upstream update will be considered an update of the same semver level for my adapter too (i.e. upstream feature update = feature update, upstream patch update = patch update). Also, because my package is an adapter, the upstream package is the only dependency. For me, it would be preferable if Greenkeeper can automatically select the correct "type" in commit messages, e.g. "feat:" for 5.7.0->5.8.0 update, and "fix:" for 5.7.0->5.7.1 update.

Proposal

We already have greenkeeper.commitMessages.dependencyUpdate and greenkeeper.commitMessages.devDependencyUpdate. Maybe we can add dependencyUpdateMajor, dependencyUpdateMinor and dependencyUpdatePatch to the list of configurable fields, and fallback to dependencyUpdate when any of these is not defined.
Or alternatively, allow dependencyUpdate to be an object, with key major, minor and patch.
And then Greenkeeper needs to determine the semver level of each update and use the corresponding commit message.