DocBook reader: Support <?asciidoc-br?>.

Closes #1236.

Note, this is a bit of a kludge, to work around the fact that xml-light
doesn't parse `<?asciidoc-br?>` correctly.  We preprocess the input,
replacing that instruction with `<br/>`, and then parse that as a line
break.  Other XML instructions are simply removed from the input stream.
Highlighting:  Let .numberLines work even if no language given.

Closes #1287, jgm/highlighting-kate#40.
Removed old haddock reader code.  Add dependency on haddock-library.

This also removes the dependency on alex and happy.
Travis:  don't need alex, happy.
Rewrote haddock reader to use haddock-library.

This brings pandoc's rendering of haddock markup in line
with the new haddock.

Note that we preserve line breaks in `@` code blocks, unlike
the earlier version.

Modified tests pass.  More tests would be good.
Finished first draft of Haddock writer.
Added haddock template.
More polish on Haddock reader/writer.
Added haddock writer tests.
Revised haddock reader tests for changes in reader.
Small fix to haddock "tables".
Haddock writer:  Only use Decimal list style.
Updated haddock writer tests.
Haddock writer:  Use _____ for hrule.

Avoids interpretation as list.