Add function to check for solid archive

[ganego]

Thank you for writing this library. I used python-unrar before, but it no longer seems to work properly.

My code behaves differently when encountering a solid archive vs a non-solid one, so I need to know when an archive is solid. For this I currently load the first couple of bytes of that rar and check for the 'solid' flag (this won't work with an SFX, but that's no issue for me).

I'd like to propose that you add a function that returns if an archive is solid.

Thank you