Adding dict support for renaming columns

[henrydingliu]

Addressing #660

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.04%. Comparing base (67f95de) to head (baccf52).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
chainladder/core/pandas.py	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   85.03%   85.04%   +0.01%     
==========================================
  Files          85       85              
  Lines        4890     4896       +6     
  Branches      627      629       +2     
==========================================
+ Hits         4158     4164       +6     
  Misses        522      522              
  Partials      210      210              

Flag	Coverage Δ
unittests	85.04% <87.50%> (+0.01%)	⬆️

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