Named subpatterns are now passed to `mb_ereg_replace_callback`.
This commit also adds a subset of the oniguruma back-reference syntax
for replacements:
* `\k<name>` and `\k'name'` for named subpatterns.
* `\k<n>` and `\k'n'` for numbered subpatterns
These last two notations allow referencing numbered groups where n > 9.
