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

src/ipc.c

@@ -594,6 +594,11 @@ static void dump_bar_config(yajl_gen gen, Barconfig *config) {
     YSTR_IF_SET(status_command);
     YSTR_IF_SET(font);
 
+    if (config->separator_symbol) {
+        ystr("separator_symbol");
+        ystr(config->separator_symbol);
+    }
+
     ystr("workspace_buttons");
     y(bool, !config->hide_workspace_buttons);