Support For Field Grouping

@wardi thoughts on adding field groups to scheming master? Here's an example of how they render by default at the moment:

It makes a few small changes:

- Adds `default_group` as an optional top-level setting on the schema (defaults to "General" if not set)
- Adds `group` as a per-field option, defaulting to the value of `default_group`.
- Adds `group_order` as an optional top-level setting on the schema to specify the display order of groups. `group_order=["General", "Contact", "License"]`. If not specified places "General" first followed by alphabetical groups.