Add metadata without bumping

[btel]

I am using cargo-edit to add metadata tags to my version string. For the moment, I am having trouble with adding metadata to version with pre-release tags (ex. 0.1.0-dev). cargo set-version -m meta will remove the pre-release and replace it with the meta tag (0.1.0+meta).

This MR will fix this behaviour to respect the dev tag, so the correct version will be 0.1.0-dev+meta.

Let me know what you think! (and thanks for making cargo-edit!)

[epage]

The relevant issue for this is #938 and the design needs to be worked out there (and how to handle if this is a breaking change) before moving on to a PR.