Moved some doc files from data-files to extra-source-files.

They aren't needed at runtime.

We keep README and COPYRIGHT in data to ensure that they'll be
available on all systems on which pandoc is installed.

Closes #1123.
LaTeX reader:  handle @{} and p{length} in tabular.

The length is not actually recorded, but at least we get a table.

Closes #1180.