Add something like a dry-run

[tmaier]

It would be great if I could call bundle exec middleman invalidate --dry-run

which would then just output list_files(options.filter) (see commands.rb#L58).

This would allow me to see if my filter regex is correct or if I did some mistake

[manuelmeurer]

Good idea! I'm a bit short on time right now, but if you or someone else would like to take a stab at implementing this, go for it! :smile_cat: