Fix directive YAML option blocks

[chrisjsewell]

Currently, YAML block are added only for code blocks:

As you can see, they work. However, they will currently be matched at any point in the cell:

This needs to be fixed, so that they are only matched if they start on the first line. Then they can be rolled out for all directives. I'm not sure what the correct grammar if for this

[chrisjsewell]

https://discuss.atom.io/t/sequential-match-rules-in-a-language-grammar/15777 https://github.com/Microsoft/vscode-textmate/issues/32