RFC: Added named regular expressions.

PCRE supports named regular expressions (ala perl/python/ruby), and they could already be used for back referencing.  This pull request adds a `get_name_table` method to `pcre.jl`, a `capture_dict` variable to `RegexMatch`es to allow access to the named captures, and minor related updates to `Regex`, `show`, etc.  It also adds a blurb in the manual.