Update nested language proposal

The examples are confusing rather than helpful.

Php/HTML is a good example.

Another great example is reStructuredText, which allows specification of a language for a code block. And Markdown now has a similar concept. Our `rstcheck` bear already uses this and lints the code blocks. Ideally this project would re-implement the rstcheck's linting of code blocks, so the splitting and spawning of linters is done inside coala instead of inside rstcheck, and the same functionality would be available for markdown without a custom bear.