Display inline images in topics

[vermiculus]

Implement the view-parts of #41.

This is still something of a proof-of-concept and is dependent on jrblevin/markdown-mode#378. Make sure you set markdown-display-remote-images to t (it could be let-bound to a Forge-customization here) and that all appropriate protocols have been added to markdown-remote-image-protocols.

[vermiculus]

From #41 (comment)

Lets see if using ![about](https://EXTERN) actually works:

Funnily enough, this is causing an error. Don't know why EXTERN is being caught here – probably being caught as real markdown even though it's in a code-block. I think it might have something to do with hiding markup, but both markdown-hide-markup and markdown-hide-markup-in-view-modes are nil.

Your note about overlays not being copied may be relevant here.

[tarsius]

Not sure why I haven't done anything about this for so long. It seems to work well for the most part.

(it could be let-bound to a Forge-customization here)

Done that in a fixup commit.

Funnily enough, this is causing an error.

But that's something that has to be addressed in markdown-mode, right? It would be nice if this would continue after failing to insert one image. (Or at least that's something worth considering).

Unless you see a reason to delay this, I'll merge it.

[vermiculus]

Testing this since it's been a while… :-)

[vermiculus]

Once I took out ![about](https://EXTERN) from my post above, still works fine :) I see no reason for delay.

Looks like markdown-regex-link-inline needs to be adjusted to avoid matching `blocks`.