Custom Contact fields

[wmnnd]

It should be possible to create and add custom fields associated with each Contact.

[wmnnd]

This should be implemented via a .data property for each contact, storing arbitrary JSON data.

• [x] Add data property in migrations
• [x] Allow querying for custom data fields
• [x] Allow import with data fields
• [x] Allow editing data property

[wmnnd]

Implemented in #85