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]
