[RFC] support build with custom (non-standard) functions

[brody4hire]

This is just an idea of how it may be possible to support developers who need to build with non-standard functions. Here is an example where I support UPPER and LOWER functions that can handle many EU characters properly: https://github.com/brodybits/sql.js/tree/custom-eu-string-functions

This idea includes some comments that I think should be considered in src/api.coffee that can help make it easier for people like myself to add custom functions in the future.

I suspect there should be a much smarter way to do this, by using an array of FuncDef structures like they do in extension-functions.c for example.

I will very likely continue with the kind of solution proposed here to support the needs of my customer, with hope for a more standardized solution (someday).