Exercises should have rough estimates of difficulty

[mawis]

Related to https://github.com/exercism/purescript/issues/92

Topic and difficulty

Core exercises unlock optional additional exercises, which can be filtered by topic an difficulty, however that will only work if we add topics and difficulties to the exercises in the track config, which is in https://github.com/exercism/purescript/blob/master/config.json

[mawis]

Currently all but one exercise are marked as "difficulty": 1. One exercise has difficulty 2.

So all exercises are shown as “easy” on the site and the student cannot get an estimate of the difficulty.