New option to render: --dirty to generate dirty test signals for testing

[bengtmartensson]

The option should generate mildly dirty test signals, that can be used for testing of decoders etc. "Emulates an Arduino with IRremote and a rookie user."

Make sense only with #251 and #252 implemented.

Possible dirts:

• Coerce the frequency to 38kHz,
• Add an amount, cf markExcess in Infrared4Arduino (50µs ?) to all pulses and subtract it from the gaps (emulating an TSMP*38),
• Round/truncate all durations to multiples of 50µs,
• "Random" variations of the duration lengths,
• Random number of repeats, simulating that the user presses the buttons for a varying length of time. Should generate a single IrSequence.
• Randomly missing ending sequence

Feedback welcome. In particular, are configuration options necessary?