This introduces the "additional_headers", "remote_user_header" and
"skip_x_access_token" config keys to allow configuring routes to
external services that require addtional headers to be set.
"remote_user_header": defines the name of a Header that will carry the
userid of the authenticated user on the outgoing request.
"additional_headers": defines a list of header names and values that will
be added to outgoing requests on matching routes.
"skip_x_access_token": when set to true the reva access token will not
be added to the outgoing request.
Needed for #206