Cookie backend

[san650]

Add the ability to configure a cookie based backend to store the preferences.

The idea is to give the user the option to use only cookie-based store, or to use the cookie-based store as a fallback for the local storage backend.

The configuration of the backend will be done via the preferences.js configuration file (see #1).

export default {
  backends: ['local', 'cookie']
};

Where the backends value will be used as follows:

• If it's not defined, use localStorage backend and cookie backend as fallback
• If it's defined as an empty array, use memory backend (plain object see #3)
• If it's ['cookie'] use cookie backend with memory as fallback
• If it's ['local'] use localStorage backend with memory as fallback

Note that memory backend is always the last fallback.