Initial alpha release of elm-test-rs

This is the first usable version of elm-test-rs, a tests runner (like elm-test) for elm 0.19.1. The code still contains a lot of debug logging to stderr in order to see what it's doing inside. Current features of elm-test-rs are:

• Initialize tests with elm-test-rs init
• Specify where the elm binary is with the --compiler option
• Pick the initial seed for fuzz tests with the --seed option
• Precise the number of fuzz runs with the --fuzz option
• Precise the number of Node workers to run the tests with the --workers option
• Basic (not polished) console | json | junit reports available

Notable missing features are a --watch mode, more human-readable failure outputs with diffs, colored console outputs and general polish for edge cases.