respond: An extra for sending messages to the printer host. (#1053)

I have made one change to `gcode.py` to support quoted parameter
values.

I have added support for the basic `M118` command (see
https://reprap.org/wiki/G-code#M118:_Echo_message_on_host). I have
also added a `RESPOND` command that takes extended parameters.
`ECHO` might be a better name than `RESPOND` but is already defined
in `gcode.py`.

Signed-off-by: Alec B. Plumb <alec@etherwalker.com>

config/example-extras.cfg

@@ -1051,3 +1051,15 @@
 # Replicape support - see the generic-replicape.cfg file for further
 # details.
 #[replicape]
+
+# Enable the "M118" and "RESPOND" extended commands.
+# [respond]
+# default_type: echo
+#    Sets the default prefix of the "M118" and "RESPOND" output to one of
+#    the following:
+#        echo: "echo: " (This is the default)
+#        command: "// "
+#        error: "!! "
+# default_prefix: echo:
+#    Directly sets the default prefix. If present, this value will override
+#    the "default_type".