Autogenerate categories.json and guides.json



The `@class` tag now can take a comma-separated list of categories.  There is no set list of categories, new ones are created as required.  The syntax looks like this:

`@class FooWidget {UI, Views, Foo Components}`

When writing guides, the first line is used at the title and the first markdown comment is used as the description:

    # Foo Components Guide
    [//]: # This guide explains how to use Foo Components