random runner

## Features

- adds random runner so a random koan is selected to be solved (needs a
  small modification of the lein plugin for `lein koan random` to work)
- on success next random koan is selected
- on failure the same koan presented again
- there is an option to skip-koan if the padawan/playful master wants to proceed to the
  next koan without providing good solution
- if multiple placeholders are present in the koan, multiple solutions
  can be typed in with ';' as delimiter
- the context of the koan is presented and considered for
  placeholders (typically when the koan file contains not fully
  implemented functions, macros)

## Known limitations

- needs a change in `lein-koan` will provide a pull a request there too

## Possible improvements

- if pull request gets accepted will create a pull request in `clojure-koans` and add a proper description of the feature to the readme