Add "contains" filter

[clenk]

This allows filtering on, for example, observed-data objects that contain a specific cyber observable.

Related: #200.

[codecov-io]

Codecov Report

Merging #203 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
+ Coverage    99.5%   99.53%   +0.02%     
==========================================
  Files          62       62              
  Lines        6527     6731     +204     
==========================================
+ Hits         6495     6700     +205     
+ Misses         32       31       -1

Impacted Files	Coverage Δ
stix2/test/test_datastore_filters.py	100% <100%> (ø)	⬆️
stix2/datastore/filters.py	98.05% <100%> (+0.07%)	⬆️
stix2/test/test_utils.py	100% <0%> (ø)	⬆️
stix2/test/test_observed_data.py	100% <0%> (ø)	⬆️
stix2/base.py	99.5% <0%> (+0.03%)	⬆️
stix2/utils.py	99.57% <0%> (+0.76%)	⬆️

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[gtback]

LGTM