Implement new BBCode tags

[greyhoof]

There should be at least one new BBCode tag (eicon) that is not implemented yet.

• [x] Implement means to download and cache images (also counts for icons)
• [x] Figure out a way to handle the load and display of images
• [x] Animate eicons if needed
• [x] Implement spoiler tag

[greyhoof]

Still need to implement [spoiler]