display docs; htl blocks; better pre copy

[mbostock]

This…

• Expands the documentation for display
• Adds html fenced code blocks
• Adds svg fenced code blocks
• Adds implicit trimming to tagged fenced code blocks

One caveat I noticed is that a fenced code block like this will not produce the desired result:

```html
<img src="muybridge/deer.jpeg" width="320" alt="A deer">
```

That is because the path to muybridge/deer.jpeg is not statically analyzed and hence not rewritten to _file. But this is a preexisting problem, e.g.:

```js
html`<img src="muybridge/deer.jpeg" width="320" alt="A deer">`
```

I don’t think this is fixable (because the DOM content is constructed dynamically and hence is not amenable to static analysis). But perhaps we should note it somewhere.

Also fixes #224.

[Fil]

Re-reading these pages I feel we should explain Prettier (maybe only by adding a link), and somehow show how to make the sparkline?