Extend the macro format supporting line breaks inside APT (multiline macro)

Macros inside an APT document can produce quite long lines making them difficult to maintain and read. (This may not be the largest problem with built-in macros, but when you attempt to add your own it can become quite problematic when using more parameters or longer values).

Example "swf macro":

%{swf|id=MyMovie|src=http://somehost/swf/myfile.swf|width=600|height=200|quality=high|play=true}

It would be nice if there was a possibility to use a syntax similar to properties files allowing to add linebreaks to a macro call, e.g.:

%{swf|id=MyMovie|src=http://somehost/swf/myfile.swf\
    |width=600|height=200\
    |quality=high|play=true}

The logic could be done like: "If a macro line ends with "\", the next line is appending the current line excluding "\" and the leading white spaces of the next line."
This should also not break any existing logic as macros end with "}" normally, so even escaping is not an issue.