text: Title: [INF] Don't require column name arguments to be strings
  Body: It occurs to me that there are methods within `pyjanitor` that require column name method arguments to be strings (specified in the docs and/or validated in the actual code).  Examples are `engineering.convert_units()` and the two methods in the finance submodule (both of which I've worked on...guilty as charged :))  
  
  Pandas does not require column names to be strings, and this could pose a big limitation for one's workflow (depending on the data set).  I'm not sure how pervasive this issue is throughout the `pyjanitor` methods; but, it seems like a discussion worth having, potentially resulting in a sweep of the current code base to remove such limitations.
  
  
  
  