These functions set up the parser object to
call the given callbacks upon tags, entities
and/or data.
The callbacks will only be called if there isn't
another tag/container/entity handler for these.
The function will be called with the parser
object as first argument, and the active string
as second.
Note that no parsing of the contents has been done.
Both endtags and normal tags are called, there is
no container parsing.
The return values from the callbacks are handled in the
same way as the return values from callbacks registered with
add_tag and similar functions.
The data callback will be called as seldom as possible with
the longest possible string, as long as it doesn't get called
out of order with any other callback. It will never be called
with a zero length string.