Make compatible with omd 1.0.0
Merge branch 'master' into 'omd'
Put the JavaScript in a HTML block
Make Code.highlight return an Omd.t value
Make comment clearer
Adapt eval & syntax highlighting to the latest omd
Basic tutorial: show the latest error message

The evaluation is done in a <div> block that is hidden.
Basic tutorial: use the right UTF-8 symbol for em-dash
99problems: Fix title & code within list
Update the lower bound for omd
Use omd 1.2.1

Fix https://github.com/ocaml/ocaml.org/issues/518