[ENH] Coalesce keyword args only

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Use keyword arguments for bfill and ffill
• Use variable args for the column names (uses _select_columns under the hood)

This PR resolves #882

PR Checklist

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Relevant Reviewers

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• @ericmjl

[codecov]

Codecov Report

Merging #887 (0d33629) into dev (36d6a6b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #887   +/-   ##
=======================================
  Coverage   95.66%   95.66%           
=======================================
  Files          19       19           
  Lines        2448     2448           
=======================================
  Hits         2342     2342           
  Misses        106      106           

[samukweku]

@pyjanitor-devs/core-devs Hi Team; I'd appreciate your feedback if using variable args is worth using here (I may be overdoing it, since I like it)

[nvamsikrishna05]

Looks good @samukweku

[ericmjl]

@nvamsikrishna05 when the tests pass, please do the honors!