Added background and border keys to the i3bar protocol.

This patch adds two new status block keys, background and border, which
define the respective colors for the status block. If not specified, the
current behavior is kept, e.g., no background / border will be drawn.

If the status block is marked urgent, the urgent color is prioritized.

fixes #2022

docs/i3bar-protocol

@@ -136,6 +136,10 @@ color::
 	when it is associated.
 	Colors are specified in hex (like in HTML), starting with a leading
 	hash sign. For example, +#ff0000+ means red.
+background::
+	Overrides the background color for this particular block.
+border::
+	Overrides the border color for this particular block.
 min_width::
 	The minimum width (in pixels) of the block. If the content of the
 	+full_text+ key take less space than the specified min_width, the block
@@ -207,6 +211,8 @@ An example of a block which uses all possible entries follows:
  "full_text": "E: 10.0.0.1 (1000 Mbit/s)",
  "short_text": "10.0.0.1",
  "color": "#00ff00",
+ "background": "#1c1c1c",
+ "border": "#ee0000",
  "min_width": 300,
  "align": "right",
  "urgent": false,