Simplify directive nodes

Currently, there’s duplicate data in directives (processing instructions and declarations):

``` json
{
  "type": "directive",
  "name": "!doctype",
  "value": "!doctype html"
}
```

I’d like to propose the following instead:

``` json
{
  "type": "directive",
  "name": "!doctype",
  "value": "html"
}
```

But it’s breaking so it will have to wait a while.
