github:export-pattern-to-repo: Replace images with placeholders before sending to repo

[GeoJunkie]

When creating patterns for the pattern library, site images should not be added. As part of running github:export-pattern-to-repo, the command needs to:

• Identify any images included in the block plattern.
• Replace those images with the equivalent size placeholder image from Placehold

This should all be done before pushing the created branch to GitHub.