Provide helpers for replacing images in post content

[tcrsavage]

Most imports involving rich text would benefit from helpers which allow for automatic replacement of image references within body content.

On first pass, this can probably be a helper function which takes in a rich text string, identifies embedded images, imports the associated image into WordPress and replaces the reference with the imported image.

The helper should probably allow whitelisting and blacklisting of specified domains so that developers have control over which images are replaced