Allow users to define the fields they want to serialise/update

[mariocj89]

Allow users to skip a field being retrieved from github. So if user don't want to display the description of repositories they can just list it as ignored.

A possible implementation is just to ignore the fields that are removed from the local config file.

At the moment, removing the description from dothub file gives the following output:

Pushing '.dothub.repo.yml' to 'dothub-sandbox/test-repo'
Changes:
- root['options']['description']
Apply changes? [Y/n]:

Even though the description is not updated