rfc5322_header: allow config file overrides
added data.headers.ini
removed inadvertent commit of config/data.headers

(missing .ini)
renamed rfc5322_header_checks -> headers

does more than RFC 5322 checks now, and is now configurable to do less as well (in
 case you want to use the plugin, but allow common misconfigurations, like the lack
of a Date header)
headers: added Date validity tests,

updated UPGRADE with deprecation notes
data.headers.ini, added more setting descriptions
updated docs/plugins/data.headers.md

Conflicts:
	UPGRADE
	config/data.headers.ini
updated config/plugins for new data.headers