Add support for further OG properties, especially :alt (alt text for WCAG)

[JPustkuchen]

Thank you very much for this super helpful library!

I was just looking for a way to retrieve og:image:alt which is relevant for the alt tag of images retrieved for WCAG.

Looking at
https://github.com/fusonic/opengraph/blob/2daa6dce84f23b1bb6d66bf03b3e9371c39cd378/src/Property.php
og:image:alt doesn't seem to be supported yet?

Maybe other properties might also make sense, so it has a wide support for what OG defines?

[JPustkuchen]

I created a PR for this, would be great if it could get merged and released, if it's fine!
#42