Add support for a custom separator symbol

This introduces a "separator_symbol" property for the i3bar configuration.
If set, the specified string will be used as a separator instead of a vertical line. Since it is an optional configuration, complete backwards compatibility is given.

fixes #1472

include/config.h

@@ -285,6 +285,9 @@ struct Barconfig {
     /** Font specification for all text rendered on the bar. */
     char *font;
 
+    /** A custom separator to use instead of a vertical line. */
+    char *separator_symbol;
+
     /** Hide workspace buttons? Configuration option is 'workspace_buttons no'
      * but we invert the bool to get the correct default when initializing with
      * zero. */