klippy: Allow each module to define their config sections

Create add_printer_objects() functions in the fan, heater, extruder,
and toolhead modules.  Create the necessary printer component objects
from this call instead of placing the code directly in klippy.py.

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

klippy/fan.py

@@ -30,3 +30,7 @@ class PrinterFan:
         self.mcu_fan.set_pwm(mcu_time, value)
         self.last_fan_time = mcu_time
         self.last_fan_value = value
+
+def add_printer_objects(printer, config):
+    if config.has_section('fan'):
+        printer.add_object('fan', PrinterFan(printer, config.getsection('fan')))