pwm_tool: Add support for high-speed PWM pin updates

The output_pin module is only capable of updating an output pin at
most once every 100ms.  Add a new pwm_tool module that is capable of
queuing updates in the micro-controller and thus allowing for much
higher update rates.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Reference.md

@@ -3127,6 +3127,26 @@ pin:
 #   parameter.
 ```
 
+### [pwm_tool]
+
+Pulse width modulation digital output pins capable of high speed
+updates (one may define any number of sections with an "output_pin"
+prefix). Pins configured here will be setup as output pins and one may
+modify them at run-time using "SET_PIN PIN=my_pin VALUE=.1" type
+extended [g-code commands](G-Codes.md#output_pin).
+
+```
+[pwm_tool my_tool]
+pin:
+#   The pin to configure as an output. This parameter must be provided.
+#value:
+#shutdown_value:
+#cycle_time: 0.100
+#hardware_pwm: False
+#scale:
+#   See the "output_pin" section for the definition of these parameters.
+```
+
 ### [static_digital_output]
 
 Statically configured digital output pins (one may define any number