add -u flag, error if variable undefined

[birchb1024]

[xyb3rt]

What is your opinion on this, @rakitzis? Can you please add some tests to trip.rc for this, @birchb1024?

[rakitzis]

Seems like a useful feature, but the change could be harmonized with the [admittedly idiosyncratic] coding style.

[rakitzis]

And yes, it could use some test coverage in trip.rc