Support creating composite indexes in migrations

[tiagoengel]

There's no support for creating composite indexes (indexes on multiple columns) in migrations, unless using raw sql.

I think the addIndex function should accept a string (like today) or an array in the column param.

The addIndex and the removeIndex functions need to be changed to support this.

The persistence.store.websql module already supports composite indexes in the createIndex function. The addIndex implementation could be very similar to that.