hangman: Implement canonical-data.json

[kytrinyx]

We want to have a standard set of test inputs and outputs for each exercise to
make it easier to port them to new languages, as well as to help keep the
exercises in sync.

The Hangman exercise can be found in the ./exercises/hangman/ directory.

The step-by-step instructions for how to compile a canonical-data.json file is
described in this section of the contributing guide.

See http://exercism.io/contribute/canonical-data/hangman for the up-to-date list of
language tracks that have an implementation of the Hangman exercise.

The tracking issue has more context.

[cmccandless]

Currently this exercise is described as an FRP exercise. This presents a couple of unique challenges to standardizing the inputs/outputs in JSON form:

• Not every language has FRP. Some can add it through 3rd-party libraries, which are usually not required for exercise test suites (though student solutions might utilize them)
• Ideally, the canonical data would, at minimum, reference FRP (or at least RP). However, defining test cases like this stretches the current format of the canonical data schema (see canonical data for react for another example of this sort of complex test case).