[ENH] Add snake option to clean_names

[ericmjl]

PR Description

Please describe the changes proposed in the pull request:

• Adds the option to convert names to snake_case from camelCase.
• Adds a bunch of tests relevant to the problem.
• References the original implementation on GitHub Gists.

This PR resolves #498.

Relevant Reviewers

• @hectormz
• @szuckerman
• @sallyhong
• @zbarry
• @jk3587

[codecov]

Codecov Report

Merging #509 into dev will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #509      +/-   ##
==========================================
+ Coverage   92.88%   92.94%   +0.06%     
==========================================
  Files           9        9              
  Lines         843      851       +8     
==========================================
+ Hits          783      791       +8     
  Misses         60       60