Implement Lua backend

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+### The `lua` backend
+
+The `lua` backend provides a flexible programming environment, allowing users to route and manipulate
+events using the Lua programming language.
+
+Every instance has it's own interpreter state which can be loaded with custom handler scripts.
+
+To process incoming channel events, the MIDIMonster calls corresponding Lua functions with
+the value (as a Lua `number` type) as parameter. To send output on a channel, the Lua environment
+provides the function `output(channel-name, value)`.
+
+Example script:
+```
+function bar(value)
+	output("foo", value / 2)
+end
+```
+
+Input values range between 0.0 and 1.0, output values are clamped to the same range.
+
+#### Global configuration
+
+The backend does not take any global configuration.
+
+#### Instance configuration
+
+| Option	| Example value		| Default value 	| Description		|
+|---------------|-----------------------|-----------------------|-----------------------|
+| `source`	| `script.lua`		| none			| Lua source file	|
+
+A single instance may have multiple `source` options specified, which will all be read cumulatively.
+
+#### Channel specification
+
+Channel names may be any valid Lua function name.
+
+Example mapping:
+```
+lua1.foo > lua2.bar
+```
+
+#### Known bugs / problems
+
+Using `output` as an input channel name to a Lua instance does not work, as the interpreter has
+`output` globally assigned to the event output function. Using `output` as an output channel name
+via `output("output", value)` works as intended.
+
+The path to the Lua source files is relative to the current working directory. This may lead
+to problems when copying configuration between installations.