Move stylemodifiers and pattern parameters machinery into Mustache engine

[geoffp]

Suggested part of #603.

@bmuenzenmeyer and I have often talked about this, as we believe that Mustache is (and probably will always be) the only engine that uses this functionality. It would be instructive to see how much complexity is subtracted from the core once this is done.

[bmuenzenmeyer]

unassigning myself - i only like to assign issues to things i plan to work on imminently

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[JosefBredereck]

Closing in favor of #1177