Schema option to specify column names

[jaidevd]

The schema should support something like

colnames:
    - col_1
    - col_2
    - col_3

which helps populate the names argument in pandas parsers.

It can also be like

colnames:
  col_1: col_a
  col_2: col_b

in which case existing column names will be overwritten by pandas after the dataframe is loaded.

It can also be a callable

colnames: !!python/name:module_name.func_name

which calls the function on the list of columns after loading the dataset.

• Implement header argument

Provision for column names as:

• list
• dictionary
• callable