Add support for %{Varnish:xid}x (X-Varnish: ..)

Change-Id: I282210e4fdb5105223c885659ea1a7c493322e29
json: output NaN as null for '!num' modifiers

Change-Id: Ib70c21b4540416d07b539a3daba608a2c6f539d7
Merge tag '1.0.1' into debian