Changed Sink.output to a property, fix README

Rather than having one method doing double-duty as getter and setter,
with ugly syntax, why not use a property?  Now the Sink's output can
simply be read and written as a boolean.

The README has been updated to show how to manipulate the new SinkConfig
objects, as well as what the output really looks like now.

README.rst

 -  Configuration is represented as a SinkConfig object
 -  SinkConfig objects can be manipulated locally and written to the
    device with one method call
+-  Allows control of whether or not the output is enabled
 
 Examples
 --------
     >>> import pdbuddy
     >>> pdbs = pdbuddy.Sink(list(pdbuddy.Sink.get_devices())[0])
     >>> pdbs.get_cfg()
-    SinkConfig(status=SinkStatus.VALID, flags=SinkFlags.NONE, v=5000, i=3000)
+    SinkConfig(status=<SinkStatus.VALID: 2>, flags=<SinkFlags.NONE: 0>, v=5000, i=3000)
     >>> print(pdbs.get_cfg())
     status: valid
     flags: (none)
 ::
 
     >>> cfg = pdbs.get_cfg()
-    >>> cfg.v = 20000
-    >>> cfg.i = 2250
-    >>> cfg.flags |= pdbuddy.SinkFlags.GIVEBACK
+    >>> cfg = cfg._replace(v=20000, i=2250)
+    >>> cfg = cfg._replace(flags=cfg.flags | pdbuddy.SinkFlags.GIVEBACK)
     >>> cfg
-    SinkConfig(status=SinkStatus.VALID, flags=SinkFlags.GIVEBACK, v=20000, i=2250)
+    SinkConfig(status=<SinkStatus.VALID: 2>, flags=<SinkFlags.GIVEBACK: 1>, v=20000, i=2250)
 
 Write the SinkConfig object to flash
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     >>> pdbs.set_tmpcfg(cfg)
     >>> pdbs.write()
     >>> pdbs.get_cfg()
-    SinkConfig(status=SinkStatus.VALID, flags=SinkFlags.GIVEBACK, v=20000, i=2250)
+    SinkConfig(status=<SinkStatus.VALID: 2>, flags=<SinkFlags.GIVEBACK: 1>, v=20000, i=2250)
 
 Requirements
 ------------

pdbuddy/__init__.py

         """Blinks the LED quickly"""
         self.send_command("identify")
 
-    def output(self, state=None):
-        """Gets or sets the state of a Sink's output
+    @property
+    def output(self):
+        """The state of the Sink's output
 
         Raises KeyError if the ``output`` command is not available on the Sink.
         Raises ValueError if an invalid output is read.
-
-        :param state: optional value of the output to set
-
-        :returns: the output state if state is None, None otherwise
         """
-        # With no parameter, return the output state
-        if state is None:
-            value = self.send_command("output")
-            if value[0] == b"enabled":
-                return True
-            elif value[0] == b"disabled":
-                return False
-            else:
-                # If unexpected text is returned, raise an exception indicating a
-                # firmware error
-                raise ValueError("unknown output state")
+        value = self.send_command("output")
+        if value[0] == b"enabled":
+            return True
+        elif value[0] == b"disabled":
+            return False
+        else:
+            # If unexpected text is returned, raise an exception indicating a
+            # firmware error
+            raise ValueError("unknown output state")
 
-        # With a parameter, set the output state
+    @output.setter
+    def output(self, state):
         if state:
             self.send_command("output enable")
         else:
     ``status`` should be a `SinkStatus` object.  ``flags`` should be zero or
     more `SinkFlags` values.  ``v`` is the voltage in millivolts, and ``i``
     is the current in milliamperes.  `None` is also an acceptible value for
-    any of the fields."""
+    any of the fields.
+    """
     __slots__ = ()
 
     def __str__(self):

test_pdbuddy/__init__.py

 
     def test_output(self):
         try:
-            self.pdbs.output(False)
-            self.assertFalse(self.pdbs.output())
+            self.pdbs.output = False
+            self.assertFalse(self.pdbs.output)
 
-            self.pdbs.output(True)
-            self.assertTrue(self.pdbs.output())
+            self.pdbs.output = True
+            self.assertTrue(self.pdbs.output)
         except KeyError:
             self.skipTest("Command output not supported")
         except ValueError: