Add support for --statement with mutual exclusion to --table and --alter

[morgo]

In our automation, we inspect statements and detect which engine to execute them with. For example, a CREATE TABLE or a DROP TABLE are applied directly, but an ALTER TABLE goes to the spirit engine.

The problem with this is, what do you do with a CREATE INDEX statement? Well, we've decided to rewrite it to an ALTER TABLE (which is possible via the TiDB parser..)

But an alternative view of the layering here is that Spirit should do this detection and be able to execute all statements. This would allow us to maintain less logic in our internal systems, since no doubt others have the same use-case and can help maintain it.

Thus, I propose adding a new command line option for --statement which is mutually exclusive to --table and --alter. We inherited --alter from gh-ost, but in future I think it's reasonable that we can default to and promote --statement instead.