Support comma separated slices

I started using this for a PHP endpoint that only accepts "arrays" as comma-separated items instead of multiple instances of the same parameter.

So currently, given a type `T`, `[]string{"a", "b"}` turns into: `?t=a&t=b`.

But it would be great to have the option of turning this into: `?t=a,b`.

I can submit a PR for this as I plan to fork and implement this for my own usage anyway.