Adds ability to retrieve avatar with WP_Post object

[oscarssanchez]

Description of the Change

This PR adds the ability to retrieve an avatar when a post object is provided (It will retrieve the post author avatar) by checking if the passed parameter is a WP_Post object and has a post_author

Alternate Designs

Benefits

Fixes #20

Possible Drawbacks

Verification Process

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues