Adds postopen, prewrite and newsub hooks that allow userland "hook
functions" to customize behavior of the database. See README for
details. A quick example:

```js
db.hooks.prewrite.add(function (op, batch) {
  if (op.type === 'put') {
    batch.add({
      type: 'put',
      key: op.value.foo,
      value: op.key,
      sublevel: fooIndex
    })
  }
})
```

More generally, this is a move towards "renewed modularity". Our
ecosystem is old and many modules no longer work because they had
no choice but to monkeypatch database methods, of which the
signature has changed since then.

So in addition to hooks, this:

- Introduces a new `write` event that is emitted on `db.batch()`,
  `db.put()` and `db.del()` and has richer data: userland options,
  encoded data, keyEncoding and valueEncoding. The `batch`, `put`
  and `del` events are now deprecated and will be removed in a
  future version. Related to Level/level#222.
- Restores support of userland options on batch operations. In
  particular, to copy options in `db.batch(ops, options)` to ops,
  allowing for code like `db.batch(ops, { ttl: 123 })` to apply a
  default userland `ttl` option to all ops.

No breaking changes, yet. Using hooks means opting-in to new
behaviors (like the new write event) and disables some old behaviors
(like the deprecated events). Later on we can make those the default
behavior, regardless of whether hooks are used.

TODO: benchmarks, tests and optionally some light refactoring.

Closes Level/community#44.

And simplify existing tests by first checking when the hook function
is called, i.e. after db has opened.