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

parser-specs/config.spec

@@ -326,8 +326,10 @@ state BINDCOMMAND:
 ################################################################################
 
 state MODENAME:
+  pango_markup = '--pango_markup'
+      ->
   modename = word
-      -> call cfg_enter_mode($modename); MODEBRACE
+      -> call cfg_enter_mode($pango_markup, $modename); MODEBRACE
 
 state MODEBRACE:
   end