Support Decimal fields (and other field types too?)

Problem/Motivation

I have a use case for this, but for a Decimal field type.

Proposed resolution

1. Add support for DecimalItem fields
2. Consider (without being premature) how to structure things to allow support to be added for other types of field
3. Recreate the module under a more suitable name: (e.g. per_domain_field?). Be aware domain_fields → already exists