added basic filter structure

routingtable/filter/actions/accept_action.go

routingtable/filter/actions/reject_action.go

routingtable/filter/filter.go

routingtable/filter/filter_test.go

routingtable/filter/from.go

routingtable/filter/prefix_list.go

routingtable/filter/term.go

routingtable/filter/term_test.go

routingtable/filter/then.go