Images from Gitter to Matrix

[remram44]

Markdown issue, follows on #7

Gitter allows the ![alt](http://image.png) syntax to insert images in messages. Even though I correctly render to <img> in HTML, Riot strips it, only rendering attachments and previews.

I don't know how to fix this. There is just no way to get Riot to render embedded images.

[remram44]

7d41034 renders images as a link. The problem is when the original image is inside a link (Gitter generates [![alt](thumb_url)](fullsize_url)), then clicking will bring you to the image and not the link.

[remram44]

Emoji gets turned into images by pymdown. I added a workaround to ignore those, but really they should be unicode.

Filed facelessuser/pymdown-extensions#19

[remram44]

82afa9d fixes emoji rendering.