Support multiple input dialects

[ryannedolan]

The CLI, based on sqlline, currently parses ANSI SQL. This turns out to be an unpopular dialect. We should probably use the MySQL dialect by default, since that is what Flink uses. It's unclear whether this is at all possible with sqlline.

At the same time, the planner will need to support the same dialects. Ideally, we'd switch from ANSI SQL to the MySQL dialect everywhere.