Confirmation field validator

Adds a new validator, with 2 use cases:

1. `confirmation: true` of a field will look at `#{field}_confirmation` for a match, and if it doesn't, will add an error
2. `confirmation: "some_other_filed"` will look at `some_other_field` for a match, and if it doesn't, will add an error

cc @angelini @cjoudrey for review, please :)