Org reader: Read inline code blocks

Org's inline code blocks take forms like `src_haskell(print "hi")` and
are frequently used to include results from computations called from
within the document.  The blocks are read as inline code and marked with
the special class `metropolis-block`.  Proper handling and execution of
these blocks is the subject of a separate library, metropolis, which is
work in progress.

This closes #1278.