Add ability to directly choose the target dbms

[wzrdtales]

When migrating generally database specific and such bound to its specific architecture and specifications like functions and procedures, there is no generic way to accomplish this task.

While creating a new language that would translate into the specific function or procedure code would be a solution, this is clearly not a task of db-migrate.

We instead will provide the ability to explicitly execute specific queries and append the capable dbms as parameter. When switching between databases, migrations aren't broken but reduced by all dbms bound actions.