Implements 'do' tag for expression statements

[mattcoley]

Requested feature: #197
Adds a do tag to evaluate an expression without printing out the result. Previously to evaluate an expression you would need to use {{ expression }}, however if this expression returned a value you were out of luck. Now you can do {% do expression %} without the side effects of what the result of expression returns.

[codecov-io]

Codecov Report

Merging #240 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #240      +/-   ##
============================================
+ Coverage      71.6%   71.64%   +0.04%     
- Complexity     1375     1380       +5     
============================================
  Files           218      219       +1     
  Lines          4331     4338       +7     
  Branches        683      683              
============================================
+ Hits           3101     3108       +7     
  Misses          996      996              
  Partials        234      234

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/hubspot/jinjava/lib/tag/TagLibrary.java	81.81% <ø> (ø)	5 <0> (ø)	⬇️
...c/main/java/com/hubspot/jinjava/lib/tag/DoTag.java	100% <100%> (ø)	5 <5> (?)

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[boulter]

nice!