Allow a range of dashboards to be specified, to run a subset of the full test suite

This is a precursor for some work splitting the travis build across multiple jobs.

You can run:

* `./cheapseats --range 0..10` to run only the first 10 dashboards.
* `./cheapseats --range 100..` to run from 100 onwards.
* `./cheapseats --range 10` to run only dashboard number 10.