display: add support for multiple displays

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

klippy/extras/display/__init__.py

@@ -7,3 +7,15 @@ import display
 
 def load_config(config):
     return display.load_config(config)
+
+def load_config_prefix(config):
+    if not config.has_section('display'):
+        raise config.error(
+            "A primary [display] section must be defined in printer.cfg "
+            "to use auxilary displays")
+    name = config.get_name().split()[-1]
+    if name == "display":
+        raise config.error(
+            "Section name [display display] is not valid. "
+            "Please choose a different postfix.")
+    return display.load_config(config)