adds the 'Incident' stub type

[pbedat]

Hello!

I'm currently working on a "export to STIX2.1" feature for a client project. Their proprietary data model defines incidents, which are also defined in the new STIX2.1 spec as a "stub type".

I'd like to add this type to your lib as a starting point for property extensions.

[codecov[bot]]

Codecov Report

Merging #53 (0c93045) into master (ec2f5c2) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   99.29%   99.30%           
=======================================
  Files          43       44    +1     
  Lines        1993     2009   +16     
=======================================
+ Hits         1979     1995   +16     
  Misses          7        7           
  Partials        7        7           

Impacted Files	Coverage Δ
data-types.go	100.00% <ø> (ø)
incident.go	100.00% <100.00%> (ø)
stix.go	99.63% <100.00%> (+<0.01%)	:arrow_up:

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[TcM1911]

Can you add an Incident in JSON format to the file testresources/all.json? If it's in this file, both JSONMarshal and JSONUnmarshal is tested.

[pbedat]

Thanks for coming back to me so quickly!

I've added the incident sample to all.json. Do you want me to add a line about this to CONTRIBUTING.md?