Add flag_meanings and flag_values to 'viirs_edr_active_fires' categories

[djhoese]

I noticed that the "confidence_cat" dataset for VIIRS EDR Active Fires didn't have flag meanings and values defined. This PR adds them and makes the DataArray more CF compliant and hopefully easier to use.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #806 into master will increase coverage by <.01%.
The diff coverage is 92%.

@@            Coverage Diff             @@
##           master     #806      +/-   ##
==========================================
+ Coverage   82.95%   82.95%   +<.01%     
==========================================
  Files         163      163              
  Lines       23610    23622      +12     
==========================================
+ Hits        19586    19596      +10     
- Misses       4024     4026       +2

Impacted Files	Coverage Δ
.../tests/reader_tests/test_viirs_edr_active_fires.py	97.21% <100%> (+0.04%)	⬆️
satpy/readers/viirs_edr_active_fires.py	90.74% <89.47%> (-2.74%)	⬇️

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[coveralls]

Coverage increased (+0.0002%) to 82.957% when pulling 0b8a6d8 on djhoese:feature-viirs-active-fires-flags into 20e2063 on pytroll:master.