[epub] Add `custom_tags: true` to Tidy options
The current release of HTML Tidy doesn't yet support the custom-tags
option, but the next official release will.
This option will tell Tidy to treat custom elements (i.e. those that contain a hyphen) as valid.
When the next version of HTML Tidy has been released and node-libtidy has been updated accordingly, update the dependencies of html-epub
and add the new option.