Support vectors in to_string

[Jomy10]

Currently, a vector would have to be wrapped in another struct that implements the ToString trait.
Ideally, we want to be able to do this:

#[table_row]
struct Row {
    id: u32,
    privileges: Vec<String>
}

And when calling the to_string method, this would output:

+----+------------+
| id | privileges |
+====+============+
| 1  | admin      |
|    | user       |
+----+------------+
| 2  | user       |
+----+------------+