.raw() should allow specifying different queries for different engines

[KernelDeimos]

I built this in my own code outside of knex:

  _wrap_knex (db, engine) {
      let syntax = engine;
      if ( syntax === 'mysql2' ) syntax = 'mysql';
      db.case = function (choices) {
          console.log('RAW? ', syntax, choices[syntax])
          return this.raw(choices[syntax]);
      };
      return db;
  }

I didn't check out how to get client directly from knex itself but that's pretty well the only thing missing. After adding this .case() method that can be used in place of .raw(), knex gains every feature of every database without much hassle from the user of the library, and it becomes possible to migrate even the trickiest of queries. I'm a little surprised this wasn't already in here.