Add withResponseBuffer() method to limit maximum response buffer size

[clue]

This changeset adds a new withResponseBuffer() method to limit the maximum response buffer size for all request methods. The default limit is now 16 MiB and can be changed with this new method.

// new: download maximum of 100 MB
$browser->withResponseBuffer(100 * 1000000)->get($url);

This is useful to prevent possible DOS attacks where the client could otherwise run out of memory when receiving a very large response body. Accordingly, this it not considered a BC break even though this might potentially affect existing consumers of this package. The 16 MiB default was chosen as a compromise to not affect most common use cases and still provide a reasonable protection against large responses.

If you're currently processing large responses, you may use the new withResponseBuffer() method to increase this limit. As an alternative, you're recommended to use streaming responses which are not affected by this limit and allow processing arbitrary responses.

Builds on top of #172 and #170
Resolves #89