[boost] [boost.log] set_open_handler() and set_close_handler() for text_multifile_backend?

I think it makes a lot of sense for these methods to be available for the text_multifile_backend as they are for the text_file_backend. My use case is that I am logging data out in html format and being able to log data out as individual html files for browsing through later on. Being able to have an html header and footer added as files are opened and closed makes a lot of sense for this backend. Jamie