Add HTMLTextDocument class

[wch]

The HTMLTextDocument class has a similar purpose to HTMLDocument, except that its contents are loaded from an HTML file.

It is meant to be used with posit-dev/py-shiny#746

This PR also makes it possible change how HTML dependencies are rendered, by setting the following:

htmltools.html_dependency_render_mode = "json"

Normally HTML dependencies are not visible when a Tag/TagList calls .render(). However, if that value is set to "json", then HTML dependencies will be rendered into the document as something like this (note that the JSON has been prettified here, but normally is not):

<script type="application/json" data-html-dependency="">
{
  "name":"strftime",
  "version":"0.9.2",
  "source":{
    "package":"shiny",
    "subdir":"www/shared/strftime/"
  },
  "script":[
    {
      "src":"strftime-min.js"
    }
  ],
  "stylesheet":[    
  ],
  "meta":[    
  ],
  "all_files":false,
  "head":null
}
</script>

Then when the HTML is read in, the HTMLDependency object can be re-created like this:

args = json.loads(x)
dep = HTMLDependency(**args)

[jcheng5]

Other than refactoring that duplicated code into a function, LGTM