Allow a regular expression package to be replaced by another package.

[aka-ao]

#59 Related to this PR, the regexp package is very slow.
I suggest creating an interface and changing it so that processing related to regular expressions can be injected externally as a dependency.

[dgoldstein0]

Since that pr, we've added lru caching to the library, which makes frequently seen user agents parse almost instantly. Is the performance with that really a problem?

I'd be open to considering faster regex implementations if it gives a large benefit, though I'd think it'd make more sense if the regex engine we use was not exposed for dependency injection - if any regex engine is missing features we need or implements a slightly different flavor of regex which ends up having observable behavior differences, that would be problematic.