Support Regexp externals

[huozhi]

Resolves #647

Changes

Support regex pattern for externals, to exclude more external libs easily.
Regexp is supported natively, this can avoid adding extra libraries to support wildcard patterns like pattern*.

Usage

 {
   externals: {
     'externaltest': 'externalmapped',
     '/\\w+-regex/': 'regexexternal',
   }
}

[guybedford]

This looks great to me, sorry for the delayed review on this @huozhi.

The implementation seems sensbile. See the comment re the matching form.

[guybedford]

Rebased, and changed the ordering in the object form.

Thanks again @huozhi for your work here.

[huozhi]

Thank you for the great advice and quick improvements! @guybedford

[styfle]

Thanks!