Support empty bracket implicit index syntax.

[mattcoley]

It was not possible previously do something like list[3:] to get the sublist of all elements from index 3 to the end, as well as list[:3], the sublist of all elements from index 0 up to index 3.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c3c310a). Click here to learn what that means.
The diff coverage is 80%.

@@            Coverage Diff            @@
##             master     #331   +/-   ##
=========================================
  Coverage          ?   71.54%           
  Complexity        ?     1580           
=========================================
  Files             ?      239           
  Lines             ?     4965           
  Branches          ?      805           
=========================================
  Hits              ?     3552           
  Misses            ?     1126           
  Partials          ?      287

Impacted Files	Coverage Δ	Complexity Δ
...va/com/hubspot/jinjava/el/ext/AstRangeBracket.java	69.49% <80%> (ø)	20 <0> (?)

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[bgoodies]

👍

[boulter]

you could use a ternary here

[mattcoley]

I need to have a better eye out for refactoring code that I come across.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.54%. Comparing base (5f9cfbb) to head (3819e7b).
⚠️ Report is 2447 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/com/hubspot/jinjava/el/ext/AstRangeBracket.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #331      +/-   ##
============================================
+ Coverage     71.51%   71.54%   +0.02%     
- Complexity     1576     1580       +4     
============================================
  Files           239      239              
  Lines          4957     4965       +8     
  Branches        800      805       +5     
============================================
+ Hits           3545     3552       +7     
  Misses         1126     1126              
- Partials        286      287       +1     

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