set validation mode in JinjavaConfig

[boulter]

Instead of making validation mode mutable on the interpreter (dubious), set it in JinjavaConfig when building the interpreter.

Improves upon #264

[codecov-io]

Codecov Report

Merging #267 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #267      +/-   ##
============================================
+ Coverage     72.22%   72.23%   +0.01%     
  Complexity     1469     1469              
============================================
  Files           231      231              
  Lines          4562     4564       +2     
  Branches        734      734              
============================================
+ Hits           3295     3297       +2     
  Misses         1009     1009              
  Partials        258      258

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/com/hubspot/jinjava/JinjavaConfig.java	84% <100%> (+1.14%)	18 <4> (+1)	⬆️
.../hubspot/jinjava/interpret/JinjavaInterpreter.java	78.31% <100%> (-0.39%)	50 <1> (-1)

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