Introduce built-in Ecto validators

Parameters should be able to support these declaratively.

I'm thinking along these lines:

```elixir
params do
  requires :name, :string, only: ~w(john mary)
  requires :profession, :string, except: ~w(student engineer)
  requires :email, :string, matches: ~r/.+@.+/
end
```

The best place to introduce this is during the schema definition step.