Haddock reader improvements.

- Correctly handle ghci sessions.
- Fixed spacing issues.
- Simplified code.
Improved CONTRIBUTING.md.
BUGS:  Added reference to CONTRIBUTING.md.
Markdown/RST writers: Only autolink absolute URIs.

This fixes a regression and closes #830.

    $ echo '<a href="x">x</a>' | pandoc -f html -t markdown
    <x>
Fixed footnote numbers in LaTeX/PDF tables.

This fixes a bug wherein notes were numbered incorrectly
in tables.  Closes #827.

Now that we are using longtable, we can just use regular
`\footnote` commands for notes, which simplifies the code
considerably.
DocBook template:  Use DocBook 4.5 doctype.
Updated tests to match new docbook template.
Markdown reader: minor code cleanup.
Shared: readDefaultDataFile: normalize the paths.

This fixes bugs in `--self-contained` on pandoc compiled with
`embed_data_files`.  The bugs affect (a) paths containing `..`, (b)
Windows, where `\` is path separator.

Closes #833.
Improved previous patch so it really solves the Windows issue.
Revert "Improved previous patch so it really solves the Windows issue."

This reverts commit e80116c813ecfc8cde094cddd36a3b083c108fd8.
Shared: Fixed readDefaultDataFile so it works on Windows.
Add slash in internal links in reveal.js.

Closes #835.
Added Text.Pandoc.Asciify utility module.

This has functions to create ascii-only versions of identifiers.
Options: Added `Ext_ascii_identifiers`.

This will force `Ext_auto_identifiers` to use ASCII only.
Set as default for github markdown.

Note: API change.
Markdown reader:  Implemented `Ext_ascii_identifiers`.

Closes #807.
README: Documented `ascii_identifiers` extension
README: Documented markdown variants (which extensions they have).
Don't convert to lowercase in email obfuscation.

Closes #839.
README:  Noted that `--self-contained` doesn't work with `--mathjax`.