Unability to leave the jack ports unconnected

When the jack stream is instanciated, the ports of the program are connected with those of the audio system. Depending on the kind of application, the developer may or may not desire this.

This is desirable, for instance, in a media player. In a musical application, it am pretty sure it goes against standard practice of Linux audio. It is common to let the user connect the program as he wishes, and let patchbay management persist the connections outside the program.

It appears there currently is no way to disable connection, beside commenting two **jack_connect** calls in [jack.c](https://github.com/andrewrk/libsoundio/blob/master/src/jack.c) and recompiling.

I believe it would be useful, while preserving default behavior, to introduce a flag to inihibit the automatic connection of ports on the Jack backend.

In the past a few people and I have requested a similar feature in the [rtaudio library](https://github.com/thestk/rtaudio/issues/60), which seems unfortunately to have fallen into deaf ears. I would appreciate a similar option in libsoundio which I believe is a better sound library overall.