Add random access support

[axelkar]

Thanks for the crate! Could you implement random-access support on the lzma block level? You could also expose block-level information so I can cache (or not) everything the way I want to in application code.

python-xz supports random access

https://sourceforge.net/p/lzmautils/discussion/708858/thread/4d71ae25/

Edit: just saw that my xz archives only have single blocks. you can keep this issue for tracking