Markdown snippet files and multimethod split-and-to-html.

A first test for markdown snippets worked. It was necessary to change
md-files to look only in src/pages and src/posts for markdown files.
This prevents all-pages from reading snippets and from crashing due to
missing meta data.

There is a first sketch for a multimethod that splits files and converts
them to html. This is neither used nor tested!
Use the new split-and-to-html in render-page.
Add the possibility to write pages in hiccup.
Write a function that returns all page and post files and refactor hiccup-files.

fix #2
Merge branch 'hiccup-pages' into snippets