Support different RDBMS LIKE syntaxes

So far, we have only considered ANSI SQL LIKE syntax for converting into regexp, but there are several others that we can consider.

1. If `Ecto.Repo` is used, use `SomeApp.Repo.config[:adapter]` to determine what adapter is used (if `config` is not available, figure out what to do)
2. Create some mechanism to validate LIKE patterns based on currently used adapter when constructing conditions
