Add support for loading images through ImageMagick as a buffer

[mcuelenaere]

This requires https://github.com/jcupitt/libvips/pull/241

[mcuelenaere]

This fixes #162.

[mcuelenaere]

Not sure how to fix the errors found by the CIs: moving the header checks into a separate function is a possible solution here I suppose?

Incidentally, #166 should also fix this I presume.

[lovell]

Fantastic, thanks for both this and the upstream libvips changes to allow it. I'll let John merge in your PR first.

The test failure is due to a high water mark I've added for cyclomatic complexity. As you say, moving this logic to C++ land will reduce the complexity here.

[mcuelenaere]

I changed the minimum version requirements to >= 8.0, as that's what John says in https://github.com/jcupitt/libvips/pull/241.

[lovell]

I updated a few #if preprocessor statements to accommodate the forthcoming libvips major version increment - see f89e9d7

Thanks again for this - I'll update the docs and provide a credit for your good work.