[FEATURE] Callback after box rendering

A new callback is added after a box is completely rendered. The
callback can be activated by setting the box_finished_callback option.

As parameters the current box, title, subtitle and image are passed.
[TASK] Improve umlaut replacement

All existing umlauts are now replaced with their matching
character combination.
[FEATURE] Allow disabling special char deletion

A new option (delete_special_chars) is introduced that allows the
special character deletion to be disabled.
Merge pull request #1 from Intera/master-patched

merging corrections into fork