Make remove_html_comments available as sitediff.yaml option

[cweiske]

It would be nice if the CLI flag --remove_html_comments could be used in the settings.yaml file, to keep the number of required CLI options minimal.

$ sitediff --verbose --debug --cached=none --remove_html_comments
Reading config file: /project/sitediff/sitediff.yaml
/sitediff/lib/sitediff/config.rb:557:in `block in load_raw_yaml': Unknown configuration key (/project/sitediff/sitediff.yaml): 'remove_html_comments' (SiteDiff::Config::InvalidConfig)
	from /sitediff/lib/sitediff/config.rb:555:in `each_key'
	from /sitediff/lib/sitediff/config.rb:555:in `load_raw_yaml'
	from /sitediff/lib/sitediff/config.rb:573:in `load_conf'
	from /sitediff/lib/sitediff/config.rb:254:in `initialize'
	from /sitediff/lib/sitediff/api.rb:19:in `new'