Feature: Add an Upsert method

[JasonTheAdams]

Often times it's useful to update existing rows if present and insert if missing. An example of this in WordPress is the update_post_meta function. It would be useful to be able to do something like this:

DB::table('flights')->upsert(
    [
        ['departure' => 'Oakland', 'destination' => 'San Diego', 'price' => 99],
        ['departure' => 'Chicago', 'destination' => 'New York', 'price' => 150]
    ],
    ['departure', 'destination'],
    ['price']
);

Arguments:

1. The records to insert or update
2. The column(s) that uniquely identify whether the record already exists
3. The column(s) to update if the record does exist

Under the hood this will probably use the ON DUPLICATE KEY UPDATE SQL syntax, as such it will be important that the identifier column(s) are either "primary" or "unique".