pkg/types: Add UnmarshallableString type
pkg/types: Add UnmarshallableString type

Allow strings to be unmarshalled for CNI_ARGS

CNI_ARGS uses types.LoadArgs to populate a struct.
The fields in the struct must meet the TextUnmarshaler interface.

This code adds a UnmarshallableString type to assist with this.

luxas/cni,murat1985/cni,mcastelino/cni,kad/cni,dcbw/cni,rosenhouse/cni,achanda/cni,tangle329/cni,tomdee/cni,containernetworking/plugins,luxas/cni,tomdee/cni,achanda/cni,rosenhouse/cni,dcbw/cni,squeed/plugins,kad/cni,containernetworking/cni,squeed/plugins,containernetworking/plugins,appc/cni,tangle329/cni,appc/cni,mcastelino/cni,murat1985/cni,containernetworking/cni