Language injections for OCamllex

[ErinvanderVeen]

It has been discussed before, but Topiary should be able to handle Language Injections (where Topiary could format a language embedded in another language). This feature was first discussed #400. We recently added ocamllex support in #573, which is the perfect opportunity to explore this further.

Ocamllex is a good candidate to experiment with language injection because it contains only a single, pre-determined, language injection (namely the (ocaml)nodes).

After we implement this issue, we could continue looking at other (more complicated) places. i.e. Nickel's doc or Markdown codeblocks.