Add a limit for the number of scanned URLs

[pjoulot]

Hello,

First, I love this package, thank you so much for your hard work.
I would love to see a feature where you can limit the number of scans. Some site are so big that the scan last forever and usually end up crashing. It would be great if we could have a limit, so the scan stops after X URLs.

For example an new argument,
--limit=500
And linkinator will scan the first 500 URLs that he's finding, and then stops and give the results for the 500 first URLs it found.