Using external filter commands to reformat HTML

We can use pandoc as a [filter][] to clean up WYSIWYG-generated HTML. Pandoc is a commandline program, but we can call it from inside Vim either using the [bang Ex command][bang], or by configuring [the `formatprg` option][formatprg] to make the `gq` operator invoke pandoc.

[filter]: http://vimdoc.sourceforge.net/htmldoc/change.html#filter
[formatprg]: http://vimdoc.sourceforge.net/htmldoc/options.html#'formatprg'
[gq]: http://vimdoc.sourceforge.net/htmldoc/change.html#gq
[bang]: http://vimdoc.sourceforge.net/htmldoc/various.html#:!