Implement JACK backend

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+### The `jack` backend
+
+This backend provides read-write access to the JACK Audio Connection Kit low-latency audio transport server for the
+transport of control data via either JACK midi ports or control voltage (CV) inputs and outputs.
+
+#### Global configuration
+
+| Option	| Example value		| Default value 	| Description		|
+|---------------|-----------------------|-----------------------|-----------------------|
+| `debug`	| `on`			| `off`			| Print `info` level notices from the JACK connection	|
+| `errors`      | `on`                  | `off`                 | Print `error` level notices from the JACK connection	|
+
+#### Instance configuration
+
+| Option	| Example value		| Default value 	| Description		|
+|---------------|-----------------------|-----------------------|-----------------------|
+| `name`	| `Controller`		| `MIDIMonster`		| Client name for the JACK connection |
+| `server`	| `jackserver`		| `default`		| JACK server identifier to connect to |
+
+Channels (corresponding to JACK ports) need to be configured with their type and, if applicable, value limits.
+To configure a port, specify it in the instance configuration using the following syntax:
+
+```
+port_name = <type> <direction> min <minimum> max <maximum>
+```
+
+Port names may be any string except for the instance configuration keywords `name` and `server`.
+
+The following `type` values are currently supported:
+
+* `midi`: JACK MIDI port for transmitting MIDI event messages
+* `cv`: JACK audio port for transmitting DC offset "control voltage" samples (requires `min`/`max` configuration)
+
+`direction` may be one of `in` or `out`, as seen from the perspective of the MIDIMonster core, thus
+`in` means data is being read from the JACK server and `out` transfers data into the JACK server.
+
+The following example instance configuration would create a MIDI port sending data into JACK, a control voltage output
+sending data between `-1` and `1`, and a control voltage input receiving data with values between `0` and `10`.
+
+```
+midi_out = midi out
+cv_out = cv out min -1 max 1
+cv_in = cv in min 0.0 max 10.0
+```
+
+Input CV samples outside the configured range will be clipped. The MIDIMonster will not generate output CV samples
+outside of the configured range.
+
+#### Channel specification
+
+CV ports are exposed as single MIDIMonster channel and directly map to their normalised values.
+
+MIDI ports provide subchannels for the various MIDI controls available. Each MIDI port carries
+16 MIDI channels (numbered 0 through 15), each of which has 128 note controls (numbered 0 through 127),
+corresponding pressure controls for each note, 128 control change (CC) controls (numbered likewise),
+one channel wide "aftertouch" control and one channel-wide pitchbend control.
+
+A MIDI port subchannel is specified using the syntax `channel<channel>.<type><index>`. The shorthand `ch` may be
+used instead of the word `channel` (Note that `channel` here refers to the MIDI channel number).
+
+The following values are recognized for `type`:
+
+* `cc` - Control Changes
+* `note` - Note On/Off messages
+* `pressure` - Note pressure/aftertouch messages
+* `aftertouch` - Channel-wide aftertouch messages
+* `pitch` - Channel pitchbend messages
+
+The `pitch` and `aftertouch` events are channel-wide, thus they can be specified as `channel<channel>.<type>`.
+
+Example mappings:
+```
+jack1.cv_in > jack1.midi_out.ch0.note3
+jack1.midi_in.ch0.pitch > jack1.cv_out
+```
+
+The MIDI subchannel syntax is intentionally kept compatible to the different MIDI backends also supported
+by the MIDIMonster
+
+#### Known bugs / problems
+
+While JACK has rudimentary capabilities for transporting OSC messages, configuring and parsing such channels
+with this backend would take a great amount of dedicated syntax & code. CV ports can provide fine-grained single
+control channels as an alternative to MIDI. This feature may be implemented at some point in the future.