Make display colors and intervals configurable

[koebi]

For now, we use blue as a standard background color, green, yellow and red for transaction statuses.
We display transactions for 1 second.
We don't show a default text on the display.

These points, and maybe some more, should be discussed.

[Merovius]

All of this should be configurable through flags, so we can experiment after deployment in a production-environment.