Some small enhancements

 * Allowing `Validator#validator(name, fn)` to be chainable
 * Setting `Field` as the context for a validator function generator
 * Allowing `required` fn to take non-strings (checkboxes return `Boolean`, breaking `trim()`)

Regarding the 2nd enhancement, this allowed me to create a `equal` validator fn. (checking that 2 fields are equal, like confirm email/password) Without some sort of context I could hook into, I could only make this happen via closure.

I've got other ideas, and I'll likely be submitting several more PRs along the way. :)