enabling colors for bmon

This commit enables colors in bmon. It adds configuration options to
let the user decide, which colors should be used. Therefor the graph_rx and graph_tx
layouts are introduced and applied respectively.

* Add graph_rx, graph_tx and layout_cfg fields.
* Setting default colors if colorized output is enabled.
* draw_table accept additional layout parameter and apply the given layout for
  the table graph.
* Apply layouts for rx, tx graph and header and statusbar.
* Add include/layout.h, which provides functions to parse, color and attribute
  strings and set the layout.
* Adding a default layout-config example in examples/bmon.conf.

[Edit: Based on original work by eri!, #PR23]

examples/bmon.conf

@@ -86,3 +86,27 @@ history day {
 	interval	= 86400.
 	size		= 60
 }
+
+layout colors {
+    color default {
+       color_pair  = {"white", "black"}
+    }
+    color statusbar {
+        color_pair = {"blue", "white", "reverse"}
+    }
+    color header {
+        color_pair = {"yellow", "black"}
+    }
+    color list {
+        color_pair = {"white", "black"}
+    }
+    color selected {
+        color_pair = {"white", "black", "reverse"}
+    }
+    color RX_graph {
+        color_pair = {"green", "black"}
+    }
+    color TX_graph {
+        color_pair = {"red", "black"}
+    }
+}