Add emphasize-lines option to jsoninclude

[jpmckinney]

Similar to the option for literalinclude in Sphinx.

https://github.com/sphinx-doc/sphinx/blob/9a30ca7da1aeffe0c21b81fc7722eb1aed1512da/sphinx/directives/code.py#L450-L456

[duncandewhurst]

+1 this would certainly be handy, though we would need to take care that the emphasised lines don't get out of sync with the source files. For example, in OCDS 1.2, we've made wholesale changes to JSON examples to reflect changes in the schema, so we'd need to make sure to update any directives that use this option accordingly.

[jpmckinney]

We can maybe consider an option that matches lines. Sphinx's literalinclude has start-at/start-after and end-at/end-before to match which lines to display; we can maybe use a similar approach for what lines to emphasize (emphasis-start-at, etc.).