ASSTags: move limiting parameters from ASSLineContents:get() into ASSLineContents:callback() as they are useful outside of :get(), also fixes some bugs in :get() along the way;  add same limiting parameters to ASSLineTagSection:callback()
ASSTags: add :modTags() as alias for :callback() to ASSLineTagSection
ASSTags: fix typo in callback methods; make callback methods return the number of processed fields; add modTags() to ASSLineContents