Feature: infer the file to be submitted

I'd like it if, as a student, I could simply run:

```bash
~/Exercism/python/two-fer $ exercism submit
```

...and it'd infer that the argument is `two_fer.py`. I think this would be big UX win. I don't know, though, if any exercises are so complex or oddly named that the solution filename couldn't be inferred. (?) If this is a problem, we could introduce an exercise metadata file. This could also help #230:

```json
"meta_data": {
"solution_file": "two_fer.py",
"test_file": "two_fer_test.py"
}
```

Open to a PR for this?