Add nested JSON syntax to the HTTPie DSL

[isidentical]

Implement nested JSON syntax to the HTTPie DSL (through JSON form notation). Resolves #78

[codecov-commenter]

Codecov Report

Merging #1224 (fe72e9e) into master (4d7d6b6) will decrease coverage by 0.57%.
The diff coverage is 94.86%.

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
- Coverage   97.28%   96.71%   -0.58%     
==========================================
  Files          67       81      +14     
  Lines        4235     5384    +1149     
==========================================
+ Hits         4120     5207    +1087     
- Misses        115      177      +62     

Impacted Files	Coverage Δ
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	31.11% <27.90%> (-68.89%)	⬇️
tests/conftest.py	77.14% <58.33%> (-9.82%)	⬇️
tests/test_ssl.py	91.01% <66.66%> (-3.93%)	⬇️
httpie/manager/__main__.py	82.35% <82.35%> (ø)
httpie/models.py	94.73% <83.33%> (-2.64%)	⬇️
httpie/output/formatters/colors.py	92.66% <88.88%> (-0.92%)	⬇️
httpie/manager/core.py	92.85% <92.85%> (ø)
httpie/manager/plugins.py	93.39% <93.39%> (ø)
httpie/cli/json_form.py	93.87% <93.87%> (ø)
... and 52 more

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