fix typo emtpy -> empty.

resolves #831 allow filtering content source by commits

resolves #1191 use value of require key on extension in playbook as default ID if id key not specified

resolves #1191 use value of require key on extension in playbook as default ID

resolves #976 allow extension to be switched off from CLI by negating value

resolves #1190 allow CLI to control order of extensions declared in playbook

add dedicated page for commits key on content source; recommend setting git.fetch_depth when using commits key