Allow themes to have a config in documentjs.json

To allow for better themeability, I'd like to consider adding the ability to pass config variables into the mustache templates from the `documentjs.json` file.

For example, let's say I want to create a theme with two options for the header nav. One black and one blue. I can set up my theme and CSS to support both, but then allow the user to define which one they want in the json file:

```
themeConfig: {
navColor: 'blue'
}
```

That value would automatically get passed into each mustache template so that I can use that variable anywhere like `layout.mustache`

```

...

```

This makes it possible for designers to create themes that other users (non-designers) can customize.

Start by tracing how documentjs.json theme settings reach the Mustache templates, especially layout.mustache. Define how the themeConfig.navColor example should be exposed to every template, and verify that a configured value can be used in the rendered navigation output.

Written by the indexing model from the issue text.