field_encoder: rewrite message using its Fields

general idea is to use json_decoder to read json and write
data into the message fields. then can:
- match against field values in message_matcher
- create a custom Payload "hello, %field_name%" using field values
field_encoder: support non-string fields. more tests

- previously only subbed Fields that were strings because of
  type-casting with `.(string)`
- more tests to cover multiple types (string, int, bool, and
  nonexistent/undefined)
Merge pull request #9 from Clever/field-encoder

field_encoder: rewrite message using its Fields