Add an HTML renderer

[keithasaurus]

Motivation

Adding HTML form generation would be a nice win. It shouldn't be terribly difficult.

Implementation

We'd need to be able to allow for customization of the generated html. Need to handle things like:

• default values
• populating form data
• rendering errors
• posting data and handling it
• files?? Maybe out of scope
• dynamic lists?

Questions

• To what extent should the validators be limited initially? Just Dict validators at root? Only string keys?
• What about field sets?
• Can we avoid adding anything at the level of defining validators? Probably.