.vimrc: Add list-whitespace-chars toggle

Add <leader>ws for toggling list-mode -- showing unprintable chars
like trailing white-space and hard-tabs.

Other changes:
* Use ":set {{flag}}!" syntax for toggling boolean variables rather
  than ":set inv{{flag}}" syntax.
* Simplify strip-trailing-whitespace helper.
.vimrc: Tweak listchars to add precedes+extends
.vimrc: Lighten guibg color for highlight SpecialKey
.vimrc: Reselect visual block after indent/outdent