Add function for getting start of day.

[mattcoley]

Adds a function today() which returns the start of today. You can optionally include a timezone today('America/New_York) to get the start of today in a timezone.

[codecov-io]

Codecov Report

Merging #247 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #247      +/-   ##
============================================
+ Coverage     71.78%   71.84%   +0.06%     
- Complexity     1400     1402       +2     
============================================
  Files           220      220              
  Lines          4402     4412      +10     
  Branches        699      700       +1     
============================================
+ Hits           3160     3170      +10     
  Misses         1002     1002              
  Partials        240      240

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/hubspot/jinjava/lib/fn/Functions.java	83.33% <100%> (+1.42%)	33 <2> (+2)	⬆️
...va/com/hubspot/jinjava/lib/fn/FunctionLibrary.java	80% <100%> (+1.42%)	2 <0> (ø)	⬇️

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