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

i3bar/include/config.h

@@ -35,6 +35,7 @@ typedef struct config_t {
     char *bar_id;
     char *command;
     char *fontname;
+    i3String *separator_symbol;
     char *tray_output;
     int num_outputs;
     char **outputs;