Custom matrix scanners

- Introduce Scanner base class for MatrixScanner
- Create new Scanner using built-in keypad module
- Allow overriding the scanner used by KMKKeyboard

docs/scanners.md

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+# Scanners
+
+Smaller boards and macro pads sometimes assign a GPIO pin to each key, rather
+than using a full matrix. Boards like this aren't compatible with the default
+matrix scanner, so you will need to swap it out with an alternative scanner.
+
+Beside the default `Matrix` scanner, KMK includes the following:
+
+
+## keypad.Keys
+
+The `keypad.Keys` scanner treats individual GPIO pins as discrete keys. To use
+this scanner, provide a sequence of pins that describes the layout of your
+board then include it in the initialisation sequence of your keyboard class.
+
+Since the `_init_sanity_check` method in the `KMKKeyboard` class will attempt
+to validate a number of settings that are no longer needed, you need to
+override it with a modified version that doesn't fail when they aren't present.
+
+
+```python
+import board
+from kmk.kmk_keyboard import KMKKeyboard
+from kmk.scanners.native_keypad_scanner import keys_scanner
+
+
+# GPIO to key mapping - each line is a new row.
+_KEY_CFG = [
+    [board.SW3,  board.SW7,  board.SW11, board.SW15],
+    [board.SW2,  board.SW6,  board.SW10, board.SW14],
+    [board.SW1,  board.SW5,  board.SW9,  board.SW13],
+    [board.SW0,  board.SW4,  board.SW8,  board.SW12],
+]
+
+
+# Keyboard implementation class
+class MyKeyboard(KMKKeyboard):
+    def __init__(self):
+        # create and register the scanner
+        self.matrix = keys_scanner(_KEY_CFG)
+
+        # copy the key coordinates from the scanner
+        self.coord_mapping = self.matrix.coord_mapping
+
+    def _init_sanity_check(self):
+        return self
+```
+
+
+## keypad.KeyMatrix
+
+The `keypad.KeyMatrix` scanner is an alternative implementation of the default
+matrix scanner using CircuitPython's builtin keypad objects. This is currently
+experimental and ***not recommended for use***.
+
+Using this scanner is similar to the `keypad.Keys` scanner. Create the scanner
+using `keypad_matrix()` instead of `keys_scanner()`.
+
+
+## `Scanner` base class
+
+If you require a different type of scanner, you can create your own by
+providing a subclass of `Scanner`. This is a very simple interface, it only
+contains a single method, `scan_for_changes(self)` which returns a key report
+if one exists, or `None` otherwise.
+