added overriding delimiter option

Hi, i added overriding delimiter option which makes it possible to have different delimiter for each of user's object. for example users array can now support hashtags with delimiter `#`, groups with delimiter `!` and more if defined

`````javascript
$("#multi-users").mention({
    delimiter: '@',
    users: [{
        username: "ashley"
    }, { 
        username: "roger"
    }, { 
        username: "frecklefart123"
    }, {
        username: "jquery",
        delimiter: "#"
    }, {
        username: "plugin",
        delimiter: "#"
    }]
});
`````
also, optional value for `key` and `name` is added. so users arrays can have any object anmes as long as they are defined as `key`, `name` and `image` settings.

`````javascript
$("#multi-users").mention({
    key: 'name',
    name: 'description',
    image: 'image_profile',
    users: [{
        name: "Sarah",
        description: "some description",
        image_profile: "http://...."
    }, ]
});
`````

a live demo is available at [here](http://bijanebrahimi.github.io/Mention.js/)