Add raw object to group in groupby filter.

[mattcoley]

If you use the groupby filter, the actual group object is a string, however in many cases you need the underlying group object. This allows you to get the object by calling group.grouperObject. I had to add a new property for backwards compatibility reasons unfortunately.

[codecov-io]

Codecov Report

Merging #218 into master will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master     #218      +/-   ##
============================================
+ Coverage     71.46%   71.48%   +0.01%     
- Complexity     1364     1365       +1     
============================================
  Files           217      217              
  Lines          4297     4303       +6     
  Branches        683      684       +1     
============================================
+ Hits           3071     3076       +5     
- Misses          985      986       +1     
  Partials        241      241

Impacted Files	Coverage Δ	Complexity Δ
.../com/hubspot/jinjava/lib/filter/GroupByFilter.java	90% <88.88%> (-1.67%)	6 <0> (+1)

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