html5 validation support for nxml-mode & js2/css tweaks

Signed-off-by: Karsten Gebbert <karsten@null2.net>
enable flyspell for markdown mode

Signed-off-by: Karsten Gebbert <karsten@null2.net>
enable nxml-mode for html files by default

Signed-off-by: Karsten Gebbert <karsten@null2.net>