Accept regex in mangle keep options

[koddsson]

Hey there 👋

I want to only keep certain function and class names in my bundles so that our error reporting software will report errors correctly when calling Function.prototype.name. Setting keep_fnames: true adds too much unwanted data.

This PR allows users to pass a regular expression to keep_fnames and keep_classnames and will only mangle class names and functions that don't match that regular expression.

Maybe there are too much redundancy in the tests since I figure the keep_fnames and keep_classnames are already being testing somewhere else. Let me know I'll delete those tests and only keep the keep_some_* tests.

✨

[fabiosantoscode]

Nice work, thank you very much!