feat(lint): restrict nesting to avoid callback hell

[dignifiedquire]

Closes #43

[RichardLitt]

LGTM

[hackergrrl]

Hiya! 👋 Pinging this PR because I noticed it breaking js-ipfs-unixfs-engine tests (causing linting to fail).

Could you talk more about the motivation behind this? Why is strictly > 4 always bad? I've found this to be situational:

db.get('foo', res => {
  db.get(res.bar, res2 => {
    net.fetch(res2.baz, res3 => {
      db.set(res3.quux, res4 => {
        assert(res4.worked)
      })
    })
  })
})

seems like a bad idea, but

module.exports = function (repo) {
  describe('exporter', function () {
    describe('basic', function () {
      it('export a large file > 5mb', (done) => {
        const hash = 'QmRQgufjp9vLE8XK2LGKZSsPCFCF6e4iynCQtNB5X2HBKE'
        const bs = new BlockService(repo)
        const ds = new DAGService(bs)
        const testExport = exporter(hash, ds)
        testExport.on('error', (err) => {
          expect(err).to.not.exist
        })
        testExport.on('data', (file) => {
          expect(file.path).to.equal('QmRQgufjp9vLE8XK2LGKZSsPCFCF6e4iynCQtNB5X2HBKE')
          file.content.pipe(concat((bldata) => {
            expect(bldata).to.exist
            done()
          }))
        })
      })
    })
  })
}

while globally being 6 levels deep, has only 2 levels at the logical test level. Is this still callback hell? (existentially: is hell subjective!?) If so, perhaps we could discuss a common pattern for dealing with these nested chai tests that tend to bulk up the indentation level.