Support images and attachments

[zefhemel]

Here's how I would envision this to work (also given how the Ghost API works I think): in your page, you can just link or embed images as any SilverBullet page, then there's a command Ghost: Share Attachments or similar, which would upload all linked attachments in the page, and replace them with URLs to the uploaded files.

This is a mechanism that can potentially be useful not just for Ghost, but for other Share plugs as well.