add debounce option

[gr2m]

follow up for hoodiehq/hoodie#202

Use case: continuously saving a text entered by a user with every key stroke. Instead of creating thousands of revisions, we want to store the current text in memory for a short time and only save after the user stopped typing for a moment (e.g. 2 seconds).

Suggested API

hoodie.store.add('doc', properties, {debounce: true})

or set a custom timeout in ms

hoodie.store.add('doc', properties, {debounce: 5000})