Add filter to convert objects to JSON.

[mattcoley]

Adds a filter to write an object as JSON string.

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master     #213      +/-   ##
============================================
- Coverage     71.37%   71.34%   -0.03%     
- Complexity     1354     1357       +3     
============================================
  Files           215      216       +1     
  Lines          4283     4289       +6     
  Branches        683      683              
============================================
+ Hits           3057     3060       +3     
- Misses          985      988       +3     
  Partials        241      241

Impacted Files	Coverage Δ	Complexity Δ
...a/com/hubspot/jinjava/lib/filter/ToJsonFilter.java	50% <50%> (ø)	3 <3> (?)

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