README: turn all extensions into headers for direct linking

Change all `**Extension: ...**` lines into headers to allow permalinking to a specific extension.
README: reduce 1:1 Topic -> Extension nesting

Reorganize some sections whose only purpose was containing a single Extension subsection.
README: blank line after headers
README: make all extension headers H4
Correctly implement capitalisation.

Using `map toUpper` to capitalise text is wrong, as e.g.
“Straße” should be converted to “STRASSE”, which is 1 character
longer. This commit adds a `capitalize` function and replaces
2 identical implementations in different modules (`toCaps` and
`capitalize`) with it.
Add plain writer testing module to .cabal file.
Merge pull request #1479 from Aelve/capitalise

Correctly implement capitalisation.
Merge pull request #1461 from cben/readme-linkable-extensions

README: linkable extensions
Merge branch 'master' of https://github.com/jgm/pandoc into minor
Slightly fix readability of main program file.
Revert a LANGUAGE pragma accidentally missed during the merge.