Add option to set pylintrc inside prospector.yml

[chocoelho]

Fixes #231

As mentioned in the issue above, there's no way to set inside prospector.yml the path for pylintrc. Instead, users have to rely on either having the pylintrc (or similar) being inside the project folder or through the command line option --pylint-config-file which can be error prone.

With this change, users can set config_file inside pylint options on prospector.yml. it'll look like:

pylint:
  options:
    config_file: /home/xpto/projects/pylintrc
  disable:
    - bad-builtin
    - too-few-public-methods
    - missing-docstring
    - star-args

Also, if users set --pylint-config-file, it'll have priority over what was set inside prospector.yml.