Adds filters for datetime functions.

[mattcoley]

Adds plus_time, minus_time, and between_times filters to operate on datetime objects/timestamp. Examples: date|plus_time('1', 'days') will give you a date 1 day in the future; begin|between_times(end, 'minutes') will give you the number of minutes between those two dates.

I also added a strtotime to convert a string and datetime format to a datetime object. For example, "2018-07-14T14:31:30+0530"|strtotime("yyyy-MM-dd'T'HH:mm:ssZ")|unixtimestamp will output 1531558890000

Fixes #56

[codecov-io]

Codecov Report

Merging #258 into master will increase coverage by 0.06%.
The diff coverage is 72.83%.

@@             Coverage Diff              @@
##             master     #258      +/-   ##
============================================
+ Coverage     71.89%   71.95%   +0.06%     
- Complexity     1408     1437      +29     
============================================
  Files           221      226       +5     
  Lines          4433     4514      +81     
  Branches        705      717      +12     
============================================
+ Hits           3187     3248      +61     
- Misses         1004     1014      +10     
- Partials        242      252      +10

Impacted Files	Coverage Δ	Complexity Δ
.../com/hubspot/jinjava/lib/filter/FilterLibrary.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...va/com/hubspot/jinjava/lib/fn/FunctionLibrary.java	81.25% <100%> (+1.25%)	2 <0> (ø)	⬇️
...hubspot/jinjava/lib/filter/BetweenTimesFilter.java	62.5% <62.5%> (ø)	5 <5> (?)
...om/hubspot/jinjava/lib/filter/MinusTimeFilter.java	66.66% <66.66%> (ø)	4 <4> (?)
...hubspot/jinjava/lib/filter/StringToTimeFilter.java	71.42% <71.42%> (ø)	4 <4> (?)
...ain/java/com/hubspot/jinjava/lib/fn/Functions.java	84.55% <77.77%> (+1.21%)	36 <2> (+3)	⬆️
...com/hubspot/jinjava/lib/filter/BaseDateFilter.java	77.77% <77.77%> (ø)	8 <8> (?)
...com/hubspot/jinjava/lib/filter/PlusTimeFilter.java	80% <80%> (ø)	5 <5> (?)
... and 4 more

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[boulter]

Do these have any precedent in Jinja? I would prefer to keep it as compatible as possible. If not, perhaps these could filters could be named with a little more descriptively like plus_time, minus_time and between_times.

[mattcoley]

The recommended way in jinja is with python commands. We could encourage developers to use Java methods like plusDays() but those seem like the only options.

[boulter]

Looks good. Can you document the temporal unit values in each filter's docs?

[mattcoley]

I changed this to support all ChronoUnit enum values and added a link at the top of each class.