Allow custom tags definition, merged into defaults

[Limess]

Allow users to merge any custom tags into the default tags.

This is useful if there's an existing tagging strategy in place.

It may also be useful to namespace the existing tags used, or at least allow an option to do so. We already use Service tags for example - but with a different use case.

[tptee]

Thanks for this! Can you run yarn build, commit the results, and push again?

[Limess]

I've ran the build, and updated the logic to not do a merge - if you're specifying custom tags you can manually reset Service and Stage yourself which seems more logical.

[tptee]

Looks good, thank you!!