There is not yet a formal spec for an approach `config.json` file or an
article `config.json` file, but I've tried to implement linting them
from the docs for approaches [1][2] and a commit [3] on the csharp
track.

[1] https://github.com/exercism/docs/blob/5509b2f12fac/building/tracks/concept-exercises.md#file-approachesconfigjson
[2] https://github.com/exercism/docs/blob/5509b2f12fac/building/tracks/practice-exercises.md#file-approachesconfigjson
[3] exercism/csharp@4069cca97782

Make `configlet lint` produce an error for a snippet with 8 newline
characters then non-newline content on the 9th line, e.g.:

    1\n2\n3\n4\n5\n6\n7\n8\n9

Note that this means we do not count lines like `wc`:

    $ printf '1\n2\n3\n4\n5\n6\n7\n8' | wc -l
    7
    $ printf '1\n2\n3\n4\n5\n6\n7\n8\n' | wc -l
    8
    $ printf '1\n2\n3\n4\n5\n6\n7\n8\n9' | wc -l
    8
    $ printf '1\n2\n3\n4\n5\n6\n7\n8\n9\n' | wc -l
    9

Try to implement these checks for `.approaches/config.json`

- If the `introduction.authors` array is non-empty, there must be a non-empty `introduction.md` file
- Any `approaches.slug` value must have a corresponding non-empty `<slug>/content.md` file
- Any `approaches.slug` value must have a corresponding non-empty `<slug>/snippet.txt` file

And these checks for `.articles/config.json`

- Any `articles.slug` value must have a corresponding non-empty `<slug>/content.md` file
- Any `articles.slug` value must have a corresponding non-empty `<slug>/snippet.md` file

Some output:

    $ configlet lint
    [...]
    The config.json 'introduction' object is present, but there is no corresponding introduction file at the below location:
    ./exercises/practice/bob/.approaches/introduction.md

    A config.json 'approaches.slug' value is 'if', but there is no corresponding content file at the below location:
    ./exercises/practice/bob/.approaches/if/content.md

    A config.json 'approaches.slug' value is 'answer-array', but there is no corresponding snippet file at the below location:
    ./exercises/practice/bob/.approaches/answer-array/snippet.txt

    A config.json 'articles.slug' value is 'performance', but there is no corresponding content file at the below location:
    ./exercises/practice/bob/.articles/performance/content.md

    A config.json 'articles.slug' value is 'performance', but there is no corresponding snippet file at the below location:
    ./exercises/practice/bob/.articles/performance/snippet.md

Example output:

    $ configlet lint
    [...]
    There is no 'approaches.slug' key with the value 'performance', but a sibling directory exists with that name:
    ../exercism-tracks/csharp/exercises/practice/reverse-string/.approaches/config.json

Appease nimpretty.