mediawiki: furl support for passing headers

While debugging, it is sometimes needed to pass to the fastcgi server
some specific header values to the PHP engine. We add the ability to
read HTTP headers from a file and inject them in the FastCGI request.

Change-Id: Ic9a3ec642043e99862ca889a419e618ead3d9fa4
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>