Add a cache option to the constructor.

[bgaillard]

For now the library cache translations using a plain Javascript object (i.e in memory), it works but it can be problematic with large translation files.

We could add a cache option transmitted to the Polyglot constructor. If this cache option is a string it allows to use a pre-defined cacheManager, if this cache option is a function it defined a custom cacheManager.

Pre-defined cache managers could be :

• MEMORY : Cache translations using a plain Javascript object
• LOCAL_STORAGE : Cache translations using the Local Storage
• SESSION_STORAGE : Cache translations using the Session Storage
• INDEXED_DB : Cache translations using IndexedDb
• WEB_SQL : Cache translations using Web SQL

Each pre-defined cache manager could also be configured using a Javascript object, for example.

var polyglot = new Polyglot(
    {
        cache : {
            type : 'LOCAL_STORAGE', 
            params : {
                key : 'myproject.i18n'
            }
        }
    }
);

Sample custom cache manager

var polyglot = new Polyglot(
    {
        cache : function(type, locale, version, data) {
            ...
        }
    }
);