content-handler rewrite
The main thing in this patch is a restructuring of download_helper which
makes each individual action for content of unknown mime types available
as its own function. The user can then configure automation on a per-
mime-type basis, with the new variable, content_handlers.
mime_type_external_handlers was removed, and replaced by a rewritten api
embodied in the variable external_content_handlers. This was done in
order to generalize the api used by that variable to make it also
useful for the new content_handlers variable. The old api was also
overcomplex. See http://bugs.conkeror.org/issue170
The api for external_content_handlers works like this:
external_content_handlers.set("application/pdf", "xpdf");
external_content_handlers.set("image/*", "feh);
The api for content_handlers works similarly:
content_handlers.set("application/pdf", content_handler_save);