Set default sort field and direction

Fixes #66, and updated readme

You can use a column as the default sort order by adding `sort` to the field:

    { fields: [
        { key: 'year', label: 'Year', sort: 'descending' }
    ] }

It will accept any truthy value for ascending order, and `'desc'`, `'descending'` or `-1` for descending order.

Doesn't handle multiple sort fields right now.