Add feature to print the expanded job matrix

[Cam Cope]

It's hard to understand whether you have configured the job matrix/includes/excludes properly without actually running the build. Please add a CLI command to generate the list of jobs that will be evaluated based upon .travis.yml.

Use case: I'm trying to create a workflow where one job checks code formatting, and then the main build + tests run against the matrix of compiler versions. The docs are unclear whether I am including/excluding jobs correctly so I would like to verify without triggering a bunch of builds through trial and error.

Related Stackoverflow question: https://stackoverflow.com/questions/54231293/test-build-matrix-expansion-of-travis