Add image size detection

[Nodachi216]

Right now I use a different bot to enforce a minimum image size but it has no flexibility. I'd like to be able to have a minimum image size most of the time but be able to ignore (or have a different minimum) based on post title.

[FoxxMD]

Great idea! And this should be pretty easy to add.

Thinking out loud:

• Implement itemIs as a first-class rule (like AuthorRule) so it can be used with and/or conditions in rules block
• Add simple contentType SubmissionState criteria for filtering based on content type IE image, text (self text)...this can already be done for reddit-hosted content using is_self and isRedditMediaDomain but not supported for external links
  • maybe some additional parsing to determine Content-Type header of external links and simplify for contentType? Or check if reddit data on submission already returns this (previews?)
• Use sharp to determine image dimensions only if...
  • Sharp dependency is present (fail if not present)
  • If metadata properties (TBD) exist on SubmissionState (don't download image if metadata properties are not present)
• Review what metadata is available from sharp to see if any other properties would be useful for filtering by (bit depth? image space?)