Add reference_options struct to contain options

[sc68cal]

This allows the addition of ON DELETE CASCADE and other options

Fixes #13

[sc68cal]

Will add tests in next session

[sc68cal]

Originally I wanted to change the jetquery.schema.Column.Reference to make it a struct, something like

pub const Reference = struct {
    table: []const u8 = null,
    column: []const u8 = null,
    on_delete: ?DeleteOptions = null,
};

But my zig-fu isn't strong enough to figure out how to turn it into a struct while still supporting the original array initialization syntax (if that's even possible in Zig), and I didn't want to make a breaking API change.