Babel 7: use transform-runtime to externalize babel helper fns

[petermikitsh]

Summary

• [X] Tell us about the problem your pull request is solving.
• [X] Are there any open issues that are related to this?

Both of the above are addressed in #56.

• [x] Is this PR dependent on PRs in other repos? No

[petermikitsh]

This reduces bundlesize from 20792 bytes to 19799 (~ 4.77%).

[petermikitsh]

@eddyystop If possible, I would prefer a patch release of this PR prior to the breaking changes planned in #55.

[eddyystop]

Thanks!

Published as v2.1.2