Allow passing in pre-existing highlighter

[shivjm]

I’m adapting an Eleventy codebase to use remark, so I set up remark-prism. Here is the build time without it:

Wrote 147 files in 7.43 seconds (50.5ms each, v0.12.1)

And then with the plugin:

Wrote 147 files in 39.14 seconds (266.3ms each, v0.12.1)

(Not all files need to run the plugin. It was 450–500ms per file where the plugin was called.)

I saw most of the time was spent in creating the highlighter. This PR makes it possible to pass one in so it doesn’t create a new one each time, like so:

import createHighlighter from "remark-prism/src/highlight";

// when configuring remark…
const highlighter = createHighlighter({});

// …
      {
        plugin: "remark-prism",
        options: {
          highlighter: highlight,
        },
      },
// …

With the change, my build times went back down to normal:

Wrote 147 files in 8.10 seconds (55.1ms each, v0.12.1)