Added lower and upper functions

[cs-christopher-carsey]

For use in functional expressions: [?lower(foo) == 'bar']

[codecov-io]

Codecov Report

Merging #164 into develop will increase coverage by 0.8%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           develop     #164     +/-   ##
==========================================
+ Coverage    97.25%   98.06%   +0.8%     
==========================================
  Files           14       14             
  Lines         1459     1445     -14     
==========================================
- Hits          1419     1417      -2     
+ Misses          40       28     -12

Impacted Files	Coverage Δ
jmespath/functions.py	98.54% <50%> (-0.97%)	:arrow_down:
jmespath/__init__.py	100% <0%> (+56%)	:arrow_up:

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[hanagiat]

Is there a reason why this PR is not yet approved and merged? Lots of people out there need it!