Add DocX files to tree.

This introduces Text.Pandoc.DocX, and its exported `readDocX` function.
Add files to cabal.

Note there is a build warning for unused `makeImagesSelfContained`
function, since there isn't yet a command-line-option to make use of it.
Integrated the docx reader into the main pandoc program.

Changes also include generalising the types of reader allowed. The
mechanism now mimics the more general output mechanism.
Add DocX automated tests.

Note this makes use of input and output files in the tests/ dir.
Reference new DocX tests in cabal file.
Add DocX tests to test-pandoc.hs
Updated README to reflect docx reader.