Support custom options

[mrombout]

Using a hidden passage users should be able to pass options to the Spiner transformer. This hidden passage should contain an XML configuration, for example:

<options>
  <linebreaks>true</linebreaks>
  <title>My book</title>
  <cover>mybook.png</cover>
</options>

The main goal for these options is to be able to produce an end result that is as close to the final production as possible. E.g. set a proper cover image for EPUB, etc.

[mrombout]

The LINEBREAKS=TRUE option shall be used in order to switch between the two different linebreak behaviour as discussed in #50.

[mrombout]

I've updated the original issue to be in XML instead of YML (for now) in order to align with the Metadata feature. Might want to change to YAML/JSON in the future, but that's not important for this story.