Use recommended extension export

[paulrobertlloyd]

Use the export format recommended by marked, which provides an extensions options value.

This means that:

const GovukHTMLRenderer = require('govuk-markdown')

marked.setOptions({
  renderer: new GovukHTMLRenderer()
})

becomes…

const govukMarkdown = require('govuk-markdown')

marked.use(govukMarkdown())

It also means options are provided to the extension itself, rather than polluting marked’s own options object, i.e.:

marked.use(govukMarkdown({
  headingsStartWith: 'xl'
}))