Add fts5-snowball extension to support many languages in full text search

[and-se]

Sqlite supports full-text search with fts5 extension, but out of the box there is only english tokenizer.

fts5-snowball extension adds support for the snowball stemmer for a variety of languages, which could serve as a good addition to the sqlite3unicode module.

Look at this PR, which fixes some crashes and adds support to sqlite < 3.20.0 (at this release fts5 extension api slightly changed).

[nalgeon]

While snowball is a great library, it's a solid external dependency. We are currently explicitly limiting this project to extensions with zero dependencies, so I'm going to close the issue for now. Will reopen later if we expand the scope.