Add PostgreSQL JSONB data type-new code.

[halybang]

Hexya changed code so much, so I update and create new PR.
See PR #15

[codecov]

Codecov Report

Merging #69 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   87.21%   87.18%   -0.04%     
==========================================
  Files          63       64       +1     
  Lines        7119     7122       +3     
==========================================
  Hits         6209     6209              
- Misses        660      663       +3     
  Partials      250      250

Impacted Files	Coverage Δ
src/models/db_postgres.go	70.5% <ø> (ø)	⬆️
src/models/fields_json_defs.go	0% <0%> (ø)

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