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/config.h

@@ -77,6 +77,7 @@ struct Variable {
  */
 struct Mode {
     char *name;
+    bool pango_markup;
     struct bindings_head *bindings;
 
     SLIST_ENTRY(Mode) modes;