Moved extractSpaces to Shared.hs

Generalised and move the extractSpaces function from `HTML.hs` to
`Shared.hs` so that the docx reader can also use it.
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.
Add DocX files to tree.
Defined strToInlines in terms of Builder.text
Converted DocX reader to internally use the builder.
Clean up of docx reader. Stage 1
Further changes to types and now passes tests
Replicated behavoir of spanTrim
Cleanup

Fixed bug where code snippets weren't handles correctly and deleted
unused functions