Mention haddock reader in cabal description.
Documented haddock reader in README.
Added stubs for haddock reader tests.

Modify tests/haddock-reader.haddock and
tests/haddock-reader.native.
Modified haddock reader test stubs so they pass, for now.
Allow simple tables to be printed as grid tables.

if other table options are disabled.

This means you can do

    pandoc -t markdown-pipe_tables-simple_tables-multiline_tables

and all tables will render as grid tables.
Haddock writer:  use 'text' builder instead of 'str'.

This articulates strings into Str, Space, allowing them to be
hard-wrapped intelligently by the writers.

This patch also fixes a bug with trailing spaces and newlines.
(See #806.)
Parsing: Better error reporting in readWith.

- Specialize readWith to String input.
- On error have it print the line in which the error occurred,
  with a caret pointing to the column.
- This should help diagnose parsing problems in LaTeX especially.
Haddock reader: improved error reporting.
Haddock reader: add the "haskell" attribute to code blocks so they are
highlighted. We reasonably assume that code blocks in Haddock markup
wont be used for anything but Haskell code.