Support placeholders and TODOs

[eemeli]

The Stage 2 entrance criteria require "Initial spec text including all major semantics, syntax, and APIs. Placeholders, TODOs, and editorial issues are acceptable".

It would be useful if ecmarkup provided some stable way of expressing placeholders and TODOs within spec text.

[michaelficarra]

AOs can have empty <emu-alg>s and algorithm steps can be 1. TODO. What more do you need?

[eemeli]

I would like for the "TODO" to get some syntax highlighting.