OR REPLACE and IF NOT EXISTS in create trigger

[mheh]

Added support OR REPLACE and IF NOT EXISTS for SQLCreateTrigger through builders.

 db.create(trigger: "foo", table: "planet", when: .before, event: .insert)
    .ifNotExists()

I made the function signature public func ifNotExists(ifNotExists: Bool = true) -> Self but that's I wasn't sure whether to just set it to true or not, so I can change that if needed.