Implement ReadLimit to optionally limit reading from response body #71

[Kleissner]

Re #71

[mna]

Hello Peter,

Thanks for the PR, makes sense again, but a couple comments:

• how would you feel to use 0 as default limit to mean "no limit" instead, so that a zero-value Options struct would work without a limit (a read limit of 0 does not make much sense, you should emit a HEAD request if you're not interested in the body)?
• as for the other PR, if you could add a unit test with a > 0 read limit that checks the resulting behaviour (e.g. if it stops reading before an <a> anchor, that anchor's URL should not be enqueued in the crawler).

Thanks,
Martin