Switch to requests to check headers before streaming content

[raincoastchris]

Fixes #299

[rom1504]

Interesting!

If you can run some benchmarks it would be ideal.

There are some options here to use robots.txt and/or this header checking to improve both the efficiency and the compliance to host preferences.

For some use cases we also want to discard early the url if it ends up not being an image, so being able to stop the connection if the header is not appropriate may be interesting there too