Implement the core functionality to allow snippets in the templates.

NOT TESTED!
Partially apply the snippet function to in-dir for use in hiccup templates.

Bind the snippet var to a version of snippet that is partially applied
to in-dir and thus takes only the name of the snippet as
argument. This allows for the use in hiccup templates.
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
Refactor md-files to use markdown-file?

fixes #3
Use the new file-ending-predicate function to refactor md-file? and clojure-file?.

fixes #3
Replace md/to-html in generate-feed with split-and-to-html.

fixes #5
Remove the debug print statement from snippet.

fixes #6
Merge branch 'snippets'