Implement python backend

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+### The `python` backend
+
+The `python` backend provides a flexible programming environment, allowing users
+to route, generate and manipulate channel events using the Python 3 scripting languge.
+
+Every instance has its own interpreter, which can be loaded with multiple Python modules.
+These modules may contain member functions accepting a single `float` parameter, which can
+then be used as target channels. For each incoming event, the handler function is called.
+
+To interact with the MIDIMonster core, import the `midimonster` module from within your module.
+
+The `midimonster` module provides the following functions:
+
+| Function			| Usage example				| Description					|
+|-------------------------------|---------------------------------------|-----------------------------------------------|
+| `output(string, float)`	| `midimonster.output("foo", 0.75)`	| Output a value event to a channel		|
+| `inputvalue(string)`		| `midimonster.inputvalue("foo")`	| Get the last input value on a channel		|
+| `outputvalue(string)`		| `midimonster.outputvalue("bar")`	| Get the last output value on a channel 	|
+| `current()`			| `print(midimonster.current())`	| Returns the name of the input channel whose handler function is currently running or `None` if the interpreter was called from another context |
+
+Example Python module:
+```
+import midimonster
+
+def in1(value):
+	midimonster.output("out1", 1 - value)
+```
+
+Input values range between 0.0 and 1.0, output values are clamped to the same range.
+
+#### Global configuration
+
+The `python` backend does not take any global configuration.
+
+#### Instance configuration
+
+| Option	| Example value		| Default value 	| Description					|
+|---------------|-----------------------|-----------------------|-----------------------------------------------|
+| `module`	| `my_handlers.py`	| none			| (Path to) Python module source file, relative to configuration file location |
+
+A single instance may have multiple `module` options specified. This will make all handlers available within their
+module namespaces (see the section on channel specification).
+
+#### Channel specification
+
+Channel names may be any valid Python function name. To call handler functions in a module,
+specify the channel as the functions qualified path (by prefixing it with the module name and a dot).
+
+Example mappings:
+```
+py1.my_handlers.in1 < py1.foo
+py1.out1 > py2.module.handler 
+```
+
+#### Known bugs / problems
+
+Output values will not trigger corresponding input event handlers unless the channel is mapped
+back in the MIDIMonster configuration. This is intentional.
+
+Importing a Python module named `midimonster` may cause problems and is unsupported.
+
+There is currently no functionality for cyclic execution. This may be implemented in a future
+release.