Global linkrefs option

[webketje]

The option would look like:

metalsmith.use(markdown({
  linkrefs: {
    google: 'https://google.be',
    metalsmith: 'https://metalsmith.io'
  }
})

The plugin would prepend a string of markdown with links outputted as

Object.entries(options.links)
  .map(([name, url]) => `[${name}]: ${url}`)
  .join('\n')

And this can be used to great effect to define link references in one place and use them everywhere, in documentation builds for example

*You can search on the [the google website][google]*
**You can go to the [metalsmith website][metalsmith]**

It must be prepended not appended, because file contents markdown refs take precedence over globally defined refs.