Implement variable border widths for pixel/normal

fixes #325

docs/ipc

@@ -274,6 +274,8 @@ name (string)::
 border (string)::
 	Can be either "normal", "none" or "1pixel", dependending on the
 	container’s border style.
+current_border_width (integer)::
+	Number of pixels of the border width.
 layout (string)::
 	Can be either "splith", "splitv", "stacked", "tabbed", "dockarea" or
 	"output".

docs/userguide

@@ -483,14 +483,26 @@ This option determines which border style new windows will have. The default is
 
 *Syntax*:
 ---------------------------------------------
-new_window <normal|1pixel|none>
+new_window <normal|1pixel|none|pixel>
 ---------------------------------------------
-
 *Example*:
 ---------------------
 new_window 1pixel
 ---------------------
 
+The "normal" and "pixel" border styles support an optional border width in
+pixels:
+
+*Example*:
+---------------------
+# The same as new_window none
+new_window pixel 0
+
+# A 3 px border
+new_window pixel 3
+---------------------
+
+
 === Hiding vertical borders
 
 You can hide vertical borders adjacent to the screen edges using