Write a generator

[snahor]

It should do the following things:

• given the name slug of an exercise, create a skeleton (we should discuss this):

exercise
+-- example.sml
+-- test_exercise.sml
+-- exercise.sml

• fetch canonical-data.json (if it exists)
• generate the test cases (we should discuss this too)

This will be helpful for new contributors.

Some exercises may be too complex for a script like this. So far, I haven't found one yet.

R exercism/discussions#155