Better-sqlite3: Support custom SQL functions

[spiffytech]

Please add support for defining custom SQL functions, like this example with the Perl SQLite driver.

[JoshuaWise]

I suppose I could add this in a future release. PRs are also welcome.

[JoshuaWise]

I've made significant progress on this feature. Now I just need to write tests, and decide how to support aggregate functions. You can view its progress here: https://github.com/JoshuaWise/better-sqlite3/pull/26.

[JoshuaWise]

With the release of v2.3.0, you can now register custom SQL functions. To learn how, read the docs.

Currently only scalar functions are supported, but I do plan to add support for aggregate functions in the near future. I'll keep this issue open until I do so.

[JoshuaWise]

With the release of v3.0.0, you can now register custom aggregate SQL functions by using generator functions.

See the docs to learn how to use this feature.

[spiffytech]

Excellent! Thank you very much!