Add sequence number also to MIDI sink names.

As already discussed in another pull request https://github.com/thestk/rtmidi/pull/6 ALSA gets unmanagable if we use the currently provided RtMidi enumeration ids as these have many issues with race conditions while other devices or clients add ports. So far, ports can only be reliably destinguished by their names. This patch adds the port number to output devices so that they also have unique names and naming is consistent with input devices.