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/config_directives.c

@@ -421,6 +421,11 @@ CFGFUN(bar_font, const char *font) {
     current_bar.font = sstrdup(font);
 }
 
+CFGFUN(bar_separator_symbol, const char *separator) {
+    FREE(current_bar.separator_symbol);
+    current_bar.separator_symbol = sstrdup(separator);
+}
+
 CFGFUN(bar_mode, const char *mode) {
     current_bar.mode = (strcmp(mode, "dock") == 0 ? M_DOCK : (strcmp(mode, "hide") == 0 ? M_HIDE : M_INVISIBLE));
 }