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.

docs/ipc

@@ -507,6 +507,8 @@ background::
 	Background color of the bar.
 statusline::
 	Text color to be used for the statusline.
+separator::
+	Text color to be used for the separator.
 focused_workspace_text/focused_workspace_bg::
 	Text color/background color for a workspace button when the workspace
 	has focus.