pcre - improve flag handling and add Re_pcre.re

* Re_pcre.re is useful to get uncompiled form
* Add multiline and anchored flags since they seem to be supported
* explictly define the types of flags we have
add substrings = Re.substrings synonym
initial re_pcre mli
Merge pull request #29 from rgrinberg/re_pcre_improve

Re_pcre improvements