H3 navigation

# Configurable Page Menu

This change modifies the config.json schema:
- Removed the `useSideMenu` option
- Removed the `tocAnchor` option
- Added a `pageMenu` option with the following properties: disable, useHeadings, returnAnchor

`$.md.config.pageMenu.disable` replaces `useSideMenu` and `$.md.config.pageMenu.returnAnchor` replaces `tocAnchor`.

`$.md.config.pageMenu.useHeadings` is a comma-delimited list of headings that should be used in the page menu and the jump links e.g. `h2,h3`.  

An example `config.json`:

```json
{
	"lineBreaks": "gfm",
	"additionalFooterText": "",
	"anchorCharacter": "#",
	"pageMenu": {
		"disable": false,
		"returnAnchor": "[top]",
		"useHeadings": "h1,h2,h3,h4"
	}
}
```

I have a few commits in there because I attempted to nest the page menu.  I realized that may not be a good idea so I manually reverted a few of those changes.  However, each menu item does have a class included that references the heading level e.g. `h2-nav` for `H2` headings and `h3-nav` for `H3` headings, etc.