colors/szakdark: tweak NonText to be like SpecialKey

The old value was too subtle to see.  Make it more like SpecialKey since
both are used for listchars and it's not too distracting.
rst2ctags: replace the docutils-based version with a custom one

The docutils-based version was simply too slow on large documents.  This
new one is considerably faster and reduces the lag when using TagBar
with a large document.
Provide shorter names for most modes in Powerline's mode indicator.
Merge branch 'powerline-short-indicators' of https://github.com/jszakmeister/vimfiles
Promote tagbar support for .rst files to core.