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/heater.py

@@ -308,3 +308,8 @@ class ControlBumpTest:
             return True
         self.heater.control = self.old_control
         return False
+
+def add_printer_objects(printer, config):
+    if config.has_section('heater_bed'):
+        printer.add_object('heater_bed', PrinterHeater(
+            printer, config.getsection('heater_bed')))