Support multiple-file submissions and add `skip` command

In order to submit exercises with multiple files, list each file that you'd like to add after the submit command:

exercism submit file1 file2 file3

The app only delivers a new problem in a language once you've submitted at least one solution to the most recent problem that was scheduled for you. The new skip command lets you bypass problems that you don't want to solve, e.g.:

exercism skip ruby food-chain