Client-side HTML 5 validations

[andrewhwaller]

Spotted an issue for this (#46) and figured I'd take a crack at it!

This PR adds a Requirable concern that checks a form's underlying class for a presence validator on the relevant field (presence_validated?) and merges required: true into the field's attributes if such a validation is detected. If required: false is passed in, that value will be respected. I added the concern to Checkbox, Input, Select, and Textarea, since those are the field types that can take advantage of required. I also added some specs covering the Requirable behavior for all four.

Note: I wasn't sure if a concern was the way to go here. Putting this logic directly in Field felt possibly weird, so I just went with a concern to keep things modular/contained.