Expose functionality via macro

This utility is great, but it can be a bit cumbersome trying to keep the types in sync in an automated way.

Right now my build process is:

1. Bring up ClickHouse locally;
2. Run any outstanding migrations;
3. In `build.rs`, connect to ClickHouse, loop through my tables (via e.g. the system tables) and run `ch2rs` on each.

What might be nicer for the last step is to have a macro like `ch2rs!([connection_info, database_filter, table_filter, whatever_other_settings])`, that expands into the relevant structs.