Adds 'with' option to schema for specifying the Field object constructor to build the field with

[patbenatar]

Sometimes certain fields in a form require custom logic. This allows you to use different Field constructors for different fields in the schema using a with option like so:

schema: {
  name: {
    title: "Full Name",
    with: MyCoolNameField
  }
}

Thoughts? If this is something we'd like to merge, I'd be happy to add tests.

[powmedia]

I like the idea; I wonder if with is the right word though. Something more self-documenting/more obvious might work better. Maybe field or fieldType if that doesn't conflict with anything else?

Would be great if you could add tests too.

[patbenatar]

@powmedia Fair enough.. I spend a lot of time in Rubyland where method calls and options lists read like sentences. I vote for field as the option name. :+1:

Will push tests up when I have a moment.

[patbenatar]

@powmedia Added tests and defaulted options.schema in createField to an empty object if undefined. A test was failing because of an undefined field schema—I don't imagine this is a common real world use case but figured defaulting was a safe bet. Let me know if you have a better idea here.

Sorry about the whitespace changes littering the commit, my editor strips trailing whitespace—I can fix that if it's a problem.