Allow adding custom commands to the roboRIO target robotCommand file

[calcmogul]

Currently, GradleRIO provides no method for adding custom commands to robotCommand, and it overwrites robotCommand so they can't be added through a side channel. This feature would be useful for hooks before the robot program starts (e.g., setting RT capability, compressing log files, setting environment variables).

[ThadHouse]

This is possible now, but needs to be better documented. I'm going to leave this open until I can do so.