Implement P command

The new prepare_write method provides a Python interface to the P
command.  If only one sector needs to be prepared, the end sector number
can be omitted.

alpaca_isp/__init__.py

         self._send_command("R {} {}\r\n".format(start, count).encode("utf-8"))
         return self._uart.read(count)
 
+    def prepare_write(self, start, end=None):
+        """Prepare the the given flash sector(s) for write operations
+
+        If end is not specified, only the start sector is prepared.
+        """
+        if end is None:
+            end = start
+        self._send_command("P {} {}\r\n".format(start, end).encode("utf-8"))
+
 
 class ReturnCode(Enum):
     """LPC ISP return codes