Add DRYRUN flag to semver commands to allow checking tags without creating them

[oliviabarrick]

This is useful to be able to update files and create a custom release commit that includes the version before tagging it.

Example: https://github.com/justinbarrick/fluxcloud/blob/master/Makefile#L9-L10

[markchalloner]

I like this idea.

I think the trigger would be better as a named argument e.g. git-semver patcch -d rather than an environmental variable, as it is then can be documented in the usage.

If you agree then please rebase on latest master (has named arguments to use as an example).

Thanks!

[oliviabarrick]

Should be changed now! I'm supporting both --dryrun and -d for clarity.

[markchalloner]

Looks good thanks for the PR.