findemails - library for extracting emails from text

I didn't find a library (or snippet, really) for this when I went looking, so here is my solution.

Remember the Go proverb:

A little copying is better than a little dependency.

I'm aware of how hard it is to validate email addresses against RFC5322 using regular expressions. This library is focused on practical cases, not 100% spec implementation. Please include real-world examples of problems with issues and PRs.