Add support for raw html input in compose window to work around shortcomings of the markdown processor

[ford--prefect]

When composing a post or comment, the editor lacks the functionality (at leas I did not find anything…) to insert raw html.
Having this would allow working around some shortcomings of the MarkDown processor (e.g. inability to include videos).
On the other hand, this gives users a lot more power over the displayed content, so one might want to sanitize the input or even consider this feature as too dangerous.
I'm interested in your perspective on this and other possible solutions to the above mentioned issues.