Make pango markup in mode names optional with a flag.

This introduces the flag "--pango" on the mode config directive to
explicitly enable pango markup for mode names. Not setting this will
cause the mode name to be rendered as is.
This fixes a regression in 4.11 where mode names containing characters
such as '<' would break user's configs as they didn't escape these
characters.

fixes #1992

docs/ipc

@@ -717,11 +717,15 @@ This event consists of a single serialized map containing a property
 This event consists of a single serialized map containing a property
 +change (string)+ which holds the name of current mode in use. The name
 is the same as specified in config when creating a mode. The default
-mode is simply named default.
+mode is simply named default. It contains a second property, +pango_markup+, which
+defines whether pango markup shall be used for displaying this mode.
 
 *Example:*
 ---------------------------
-{ "change": "default" }
+{
+  "change": "default",
+  "pango_markup": true
+}
 ---------------------------
 
 === window event