Add `magick.new` and `magick.new_image` about magick

[leafo]

magick currently doesn't have any drawing functions, so making a new image isn't that useful. I'm not opposed to adding those kinds of functions though.

This library is an FFI binding to ImageMagick. We can easily introduce any function implemented by ImageMagick's C interface. Here are a bunch: http://www.imagemagick.org/api/magick-image.php

You just need to copy the signature of the function into the statement on the top of init.moon and then you can access it: https://github.com/leafo/magick/blob/master/magick/init.moon#L6

I'm not familiar with the internal drawing API, but maybe if you dig around you can find something