Add default and required options + package.json update

Hi Ian,

This is my submit for https://github.com/segmentio/prompt-for/issues/5

I update the package.json, change `async` for `async-each-series`. Exactly the same but really smaller footprint (the `eachSeries` method is extracted)

I add the default and required options and update the Readme.

I tried to mimic your code style and I'm pretty happy with the result. I hope you'll like it.

Just a remark:

Why do you always invert all conditional tests?
`'array' == typeof schema`
https://github.com/segmentio/prompt-for/blob/master/lib/index.js#L30

`false !== options.pad`
https://github.com/segmentio/prompt-for/blob/master/lib/index.js#L44

Another remark:

You debug `require('debug')('metalsmith-prompt')`
https://github.com/segmentio/prompt-for/blob/master/lib/index.js#L3

I think it's better to use `require('debug')('prompt-for')`

What do you think?

Thanks,
Jérôme