Lint all JavaScript in markdown files by default

ESLint supports the notion of "processors" which allows us to extract JS from markdown files and lint it to ensure it is in standard style. https://eslint.org/docs/user-guide/configuring#specifying-processor

That would let us eliminate this whole section of the readme which describes how to install eslint-plugin-markdown, run standard with the --plugin flag. In the end you need to actually run standard twice if you do this: standard && standard --plugin markdown '**/*.md'

I think we can do better. Let's just do the right thing out of the box.

We should probably do the same for