Add filter hooks and modules

Adds a filter hook that happens synchronously after the content document has been parsed, but before the afterParsed hook that many modules use. This allows for filtering out or changing parts of the DOM before it is processed by other hooks.

Current modules in filter are:

• WhiteSpaceFilter - remove whitespace when possible. From https://gitlab.pagedmedia.org/tools/pagedjs/merge_requests/115
• CommentsFilter - remove all comments.
• ScriptsFilter - remove all script elements. Closes https://gitlab.pagedmedia.org/tools/pagedjs/merge_requests/97
• UndisplayedFilter - Mark elements set to display none as undisplayed. Closes https://gitlab.pagedmedia.org/tools/pagedjs/merge_requests/86

Adds the Clear-Cut library to handle css rule specificity: https://github.com/atom/clear-cut