Drop indented

[rwxrob]

It's just the same as fenced and is harder to maintain properly. Better to just fence it all off and give it a syntax class if needed. In fact, fenced is all that is ever really needed. We should allow literally anything after the fence (like Pandoc which turns {note} into a class. This will allow the use of syntaxed fence blocks to be used for divisions as well as actual language classes. This is what Myst allows for "admonitions" even though they should probably be done within footnotes instead.

We won't be specifying what to do with what comes after the initial fence at all. That is entirely up to KEG apps to decide and greatly differs on how they will be rendered. However, we do want to caution about putting to much actual paragraph style text in such things since most Markdown engines just won't render it at all.