Add progress callback parameter to run method

[mattt]

Follow up to #106

This PR adds a progress callback parameter to the run method. The function is called with the prediction initially after creation, after each polling update, and after completion.

const model = "stability-ai/stable-diffusion:27b93a2413e7f36cd83da926f3656280b2931564ff050bf9575f1fdf9bcd7478";
const input = {
  prompt: "a 19th century portrait of a raccoon gentleman wearing a suit",
};
const output = await replicate.run(model, { input }, (prediction) => {
  console.log(prediction)
});

This PR also fixes a bug in the implementation of the wait method's stop parameter, where the callback wasn't called after the first polling request.