toolhead: Report the current extruder from the get_status() method

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

docs/Command_Templates.md

@@ -142,8 +142,9 @@ The following are common printer attributes:
 - `printer["gcode_macro <macro_name>"].<variable>`: The current value
   of a gcode_macro variable.
 - `printer.<heater>.temperature`: The last reported temperature (in
-  Celsius as a float) for the given heater. Available heaters are:
-  `heater_bed` and `heater_generic <config_name>`.
+  Celsius as a float) for the given heater. Example heaters are:
+  `extruder`, `extruder1`, `heater_bed`, `heater_generic
+  <config_name>`.
 - `printer.<heater>.target`: The current target temperature (in
   Celsius as a float) for the given heater.
 - `printer.pause_resume.is_paused`: Returns true if a PAUSE command
@@ -152,6 +153,10 @@ The following are common printer attributes:
   toolhead relative to the coordinate system specified in the config
   file. It is possible to access the x, y, z, and e components of this
   position (eg, `printer.toolhead.position.x`).
+- `printer.toolhead.extruder`: The name of the currently active
+  extruder. For example, one could use
+  `printer[printer.toolhead.extruder].target` to get the target
+  temperature of the current extruder.
 
 The above list is subject to change - if using an attribute be sure to
 review the [Config Changes document](Config_Changes.md) when upgrading