Custom matrix scanners

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

kmk/scanners/__init__.py

@@ -0,0 +1,12 @@
+class Scanner:
+    '''
+    Base class for scanners.
+    '''
+
+    def scan_for_changes(self):
+        '''
+        Scan for key events and return a key report if an event exists.
+
+        The key report is a byte array with contents [row, col, True if pressed else False]
+        '''
+        pass