Docx reader: Add ParIndentation type to parser.

This lets us keep more information about the indentation, and act
accordingly in the reader.
Docx reader: Make use of new ParIndentation info.

Here, when hanging indents are greater than or equal to left indents, we
don't set it to block quote. Such indents are frequently used in
academic bibliographies. (Thanks to Caleb McDaniel.)
Docx reader: Add tests for hanging indent handline.

We want to treat it as a plain paragraph if the hanging amount is
greater to or equal to the left indent---i.e., if the first line has
zero indentation. But we still want it to be a block quote if it starts
to the right of the margin. Someone might format verse with wrapping
lines with a hanging indent, for example.
Merge pull request #1386 from jkr/hanging_indent

Fix hanging indent behavior
Rewrote normalize for efficiency. (Closes #1385.)

* Added normalizeInlines, normalizeBlocks.
* Type signature is now more narrow, `Pandoc -> Pandoc` instead of
  `Data a :: a -> a`.  Some users may need to change their uses of
  `normalize` to the newly exported `normalizeInlines` or
  `normalizeBlocks`.