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

include/bindings.h

@@ -25,7 +25,7 @@ const char *DEFAULT_BINDING_MODE;
  */
 Binding *configure_binding(const char *bindtype, const char *modifiers, const char *input_code,
                            const char *release, const char *border, const char *whole_window,
-                           const char *command, const char *mode);
+                           const char *command, const char *mode, bool pango_markup);
 
 /**
  * Grab the bound keys (tell X to send us keypress events for those keycodes)