added get_subquery() and add_imported_fields, fixed soft-delete and join demos

[romaninsh]

Thanks to get_subquery method, creating expressions for related tables is now much easier:

// Link Users with Orders and aggregate sum(total) into "orders_total"
users = users
    .with_many("orders", "user_id", || Order::table())
    .with_expression("orders_total", |t| {
        let o = t.get_subquery::<Order>("orders").unwrap()
        o.sum(o.total()).render_chunk()
    });

For with_one relation, there is also a new field for importing fields as-is.

users = users
    .with_one("role", "role_id", || Role::table())
    .with_imported_fields("role", &["name", "permission"]);

Oh and has_one -> with_one, and has_many -> with_many - for consistency!

You can also display table now:

println!("Table: {}", &table);