Streaming parsing?

It would be really cool if this library supported streaming parsing of a message. One thing I thought of was accepting an fopen resource or Guzzle stream resource as input, and returning an iterator that returns Guzzle stream resources that are just [LimitStreams](https://github.com/guzzle/streams/blob/master/src/LimitStream.php) over the original data. Then the usage could become something like this:

``` php
$parts = new PartIterator(fopen('http://example.com/huge_multipart_body', 'r'));

foreach ($parts as $part) {
var_dump($part['headers']);
while (!$part['content']->eof()) {
echo $part->read(1024);
}
}
```

In order to get an array of parts, you'd need to provide a sort of helper function with the caveat that it loads the entire message into memory (something like this could be on the iterator):

``` php
public function toArray()
{
$parts = [];
foreach ($this as $part) {
$parts[] = ['headers' => $part['headers'], 'content' => (string) $parts['content']);
}

return $parts;
}
```