gcode_macro: Add the ability to define custom g-code macros

Add the ability to add a custom g-code command that in turn executes
one or more configured g-code commands.

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

config/example-extras.cfg

@@ -582,6 +582,14 @@
 #   measurements must be provided.
 
 
+# G-Code macros (one may define any number of sections with a
+# "gcode_macro" prefix).
+#[gcode_macro my_cmd]
+#gcode:
+#   A list of G-Code commands (one per line) to execute in place of
+#   "my_cmd". This parameter must be provided.
+
+
 # Replicape support - see the generic-replicape.cfg file for further
 # details.
 #[replicape]