Add an :external filter to run an external program.

The filter assumes that the external program takes its
input from stdin and sends the processed input to
stdout.
Minify HTML using the external tool htmlcompressor.

htmlcompressor can be installed with

    brew install htmlcompressor
Revert "Minify HTML using the external tool htmlcompressor."

This reverts commit 1d5eb1bcd7443c9a7c05daec2fe7e10c39c4c5b6.