Extract tables and joins

[mpr1]

Requesting adding a feature "table/col relations" adding a function to extract joining tables, join types and join keys. This makes this package wonderful.

[klahnakoski]

@mpr1 thank you for this request. There is a pull request (https://github.com/klahnakoski/mo-sql-parsing/pull/26) that uses this parser output to pull some of the high level features from the parsed SQL. I am not sure if it still works, but I keep it open to remind me it can be useful.

Also see https://github.com/klahnakoski/mo-sql-parsing/issues/48