This commit changes nghttpx's mruby script handling.  Previously we
have 2 options to specify the mruby script file to be run on request
and on response.  Now they are merged into 1 option, namely
--mruby-file.  It now must return object.  On request, the object's
on_req(env) method is invoked with env object.  Similarly, on
response, the object's on_resp(env) method is invoked.  The
specification of Env object has not changed.