Using `#[template(source = "…")]` is tiresome, because you have to
escape the input. That makes it difficult to read, too. Using
`#[template(path = "…")` can be tiresome, too, if the code is small
and/or if you just want to prototype something.
This PR adds a third option to supply the source of a template: You can
supply the source in the comments of a template. This works similar to
doctest. Any <code>```rinja```</code> block gets extracted, and the
combined blocks are the source code of the template.