Handle images in CLI lock folder

[cuibonobo]

The CLI currently opens a temp folder where the user can drop images and link to them from their MarkDown file. However, we're not doing anything with the images at the moment and they are automatically deleted when the temp folder is cleared.

When a post is committed:

• Parse the MarkDown for any relative image links
• Verify that the same images exist in the temp folder
• Create an attachment that saves the filename and other relevant data
• Replace the URL with an attachment ID created from a file hash

When a post is rendered:

• Use the marked renderer to replace the attachment ID with an absolute URL

When a post is edited:

• Parse the MarkDown for attachment IDs
• Retrieve the images by their ID and copy them to the temp folder
• Replace the attachment ID with the file name in the temp folder

When an edited post is committed:

• The same steps as when a post is committed, but attachments won't be created if they already exist