Shortcode rewrite, take 2

This commit contains a restructuring and partial rewrite of the shortcode handling.

Prior to this commit rendering of the page content was mingled with handling of the shortcodes. This led to several oddities.

The new flow is:

1. Shortcodes are extracted from page and replaced with placeholders.
2. Shortcodes are processed and rendered
3. Page is processed
4. The placeholders are replaced with the rendered shortcodes

The handling of summaries is also made simpler by this.

This also introduces the distinction between shortcodes that need further processing and those who do not:

* `{{< >}}`: Typically raw HTML. Will not be processed.
* `{{% %}}`: Will be processed by the page's markup engine (Markdown or (infuture) Asciidoctor)

The above also involves a new shortcode-parser, with lexical scanning inspired by Rob Pike's talk called "Lexical Scanning in Go",
which should be easier to understand, give better error messages and perform better.

The new parser supports nested shortcodes. Two related design choices:

* The shortcodes will be rendered individually, so If both `{{< >}}` and `{{% %}}` are used in the nested hierarchy, one will be passed through the page's markdown processor, the other not.
* To avoid potential costly overhead of always looking far ahead for a possible closing tag, this implementation looks at the template itself, and is branded as a container with inner content if it contains a reference to `.Inner`

Fixes #565
Fixes #480
Fixes #461

And probably some others.