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 remote-tracking branch 'upstream/master'

Conflicts:
	src/Text/John/Writers/FB2.hs
	src/Text/John/Writers/Markdown.hs
	tests/Tests/Writers/Plain.hs