text: Title: pyjanitor version of .assign() or mutate()
  Body: Currently there are a few different pyjanitor functions to deal with adding/modifying columns.
  
  - add_column() allows you to add a column, but doesn't accept lambda functions like join_apply()
  - transform_column() allows you to transform an existing column, but also doesn't accept lambda functions.
  - join_apply() accepts lambda functions, but doesn't allow you to transform an existing column. It also has an API that is slightly different from all other column editing functions. join_apply() uses (func, column_name) as opposed to (column_name, func).
  
  In general as I've been using pyjanitor I assumed there would be something along the lines of:
  
  ```python
  df.mutate('a_plus_b', lambda x: x.a + x.b)
  ```
  
  Is there a way to either add lambda functionality to add_column() and transform_column(), or add a function like mutate() that does this?