New property `multiSort` added. If it's false (default), everything works like before

However when it's set to true, you can sort data by several selected columns. The most recent selected column has the smallest precedence.

The generated request looks like this: `?sort=id|desc,name|asc,email|asc`

Of course to get this working, you'll need a backend which is able to parse the above format.

Please note that this commit breaks backwards compatibility *a bit*. Now if you want to initialise the sort-order property, it should be done with an array like this. Just wrap your previous object in an array.

```
[{
    field: 'name',
    direction: 'asc'
}]
```