i3bar: set markup per block

Add `markup` to the i3bar protocol as a block member.

This is a string that determines how the block should be parsed as
markup. "pango" indicates the block should be parsed as Pango markup.
"none" indicates the block should not be parsed as markup.

libi3/string.c

@@ -178,6 +178,13 @@ bool i3string_is_markup(i3String *str) {
     return str->is_markup;
 }
 
+/*
+ * Set whether the i3String should use Pango markup.
+ */
+void i3string_set_markup(i3String *str, bool is_markup) {
+    str->is_markup = is_markup;
+}
+
 /*
  * Returns the number of glyphs in an i3String.
  *