Configurable field fetching

[ransford]

Describe the solution you'd like Instead of hard-coding fields like HL7's PRT-10 and OBX-18 into decoders, load "fields that are of interest" information from a configuration parameter (e.g., a configuration file).

Ship with several stock configurations that capture the current behavior.

Additional context This would make it easier to experiment with Tapirx and for manufacturers/vendors/explorers to build their own configurations without having to modify source code.