Add pygments_lexer key to kernelspec
Use kernelspec metadata for higlighting in nbconvert

Closes gh-6619
Deprecate NbConvertBase.default_language config option
Wording change as suggested by @minrk
Fix attribute name
Fallback to plain text if higlighting lexer not found.
Merge pull request #6720 from takluyver/kernelspec-pygments-lexer

Respect kernelspec metadata for syntax highlighting in nbconvert