Configurable step pulse width

[jcoffland]

The Buildbotics controller is hard coded to output 2uS step pulses. Some drivers require longer widths. 2.5uS in one case. This enhancement would add a configurable option to the user interface which would allow setting the pulse width with in a range (say 2uS to 10uS). Changes to this value would affect the maximum step rate (1 / (pulse_width * 2)). Other configuration options would be automatically limited.