Additional schemas

[clenk]

Fixes #43. Note that to use a schema for a custom observable object type or custom observable extension, you have to have a schema for observed-data, but it can be an empty object.

[codecov-io]

Codecov Report

Merging #47 into master will increase coverage by 0.43%.
The diff coverage is 98.21%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage    88.2%   88.64%   +0.43%     
==========================================
  Files          29       29              
  Lines        2577     2597      +20     
==========================================
+ Hits         2273     2302      +29     
+ Misses        304      295       -9

Impacted Files	Coverage Δ
stix2validator/util.py	75.51% <ø> (-1.42%)	⬇️
stix2validator/scripts/stix2_validator.py	0% <ø> (ø)	⬆️
stix2validator/test/indicator_tests.py	100% <100%> (ø)	⬆️
stix2validator/test/observed_data_tests.py	100% <100%> (ø)	⬆️
stix2validator/validator.py	65.77% <100%> (+2.99%)	⬆️
stix2validator/test/__init__.py	95% <83.33%> (ø)	⬆️

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[gtback]

LGTM