Add :Fold, :Foldsearch, and :Foldcomments commands.
Merge branch 'add-tmux-syntax' of https://github.com/jszakmeister/vimfiles
Add "extensions" section in notes.txt; document Tmux syntax extension.
Fix up the lookup for machine-specific configs.

$VIMUSERFILES now incorporates $VIMUSER in it, so we no longer need to
insert that segment into VIMMACHINE_CONFIG.
Enable rst2ctags support.

It'll serve as extra motivation to fix the slowness.
Make gx work on urls in Markdown.

This also allows you to highlight some text and use ``gx`` to open it.