Separator color via config; separator width and on/off via ipc

This patch adds the following features:
1) Configure a color of the separator via config.  It is done like
   bar {
      colors {
         separator #000000
      }
   }
2) A block can have an integer entry "separator_block_width" which
   sets the width of the gap which would follow after the current block.

3) A block can have a boolean entry "separator" and if it is set
   to false, then the drawing of the separating line would be disabled.

include/config.h

@@ -267,6 +267,7 @@ struct Barconfig {
     struct bar_colors {
         char *background;
         char *statusline;
+        char *separator;
 
         char *focused_workspace_border;
         char *focused_workspace_bg;