Add support for capturing group in tag inclusion.

[mgao6767]

This PR allows the use of capturing group when using regular expression to determine item inclusion by tags. If tag_include_re contains capturing group, the tags will be the concatenation of captured substrings.

For example:

• In Zotero, I have tags of "-kerko-Topic 1", "-kerko-Topic 2".
• In config.toml, I set tag_include_re to "-kerko-(.*)". Enable kerko.facets.tag.
• In the web, facets will be "Topic 1" and "Topic 2", instead of "-kerko-Topic 1", "-kerko-Topic 2".

The changes pass all tests. Works as expected.