Add json_key register setting for #9

[tjhowse]

No description provided.

[codecov]

Codecov Report

Merging #22 (012ab88) into master (051fddf) will increase coverage by 0.02%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   95.56%   95.58%   +0.02%     
==========================================
  Files           6        6              
  Lines         676      747      +71     
==========================================
+ Hits          646      714      +68     
- Misses         30       33       +3     

Impacted Files	Coverage Δ
tests/test_mqtt.py	98.40% <90.00%> (-0.89%)	⬇️
modbus4mqtt/modbus4mqtt.py	93.78% <100.00%> (+1.87%)	⬆️
modbus4mqtt/version.py	100.00% <100.00%> (ø)

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