Add a flag for "normalizing" docker tags to contain only valid characters

[decarboxy]

We sometimes use git branch names as docker tag names. git branch names can have / characters, while docker tags cannot. This PR adds a new boolean option tags.normalize which replaces all ascii characters that cannot exist in a docker tag with -. The flag is off by default.

[tboerger]

We could add the normalization to the plugin code, but not as part of the main function or main function file.