add comparison operator aliases to built-in checks

[cosmicBboy]

fixes #238

This PR adds aliases for the comparison operator built-in checks for convenience

[codecov-commenter]

Codecov Report

Merging #269 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #269   +/-   ##
=======================================
  Coverage   96.61%   96.62%           
=======================================
  Files          15       15           
  Lines        1331     1364   +33     
=======================================
+ Hits         1286     1318   +32     
- Misses         45       46    +1     

Impacted Files	Coverage Δ
pandera/checks.py	98.41% <100.00%> (+0.03%)	⬆️
pandera/schemas.py	96.56% <0.00%> (-0.02%)	⬇️

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