some features i worked on

Hi @xtfxme

First of all: Thanks for this awesome script!
I played with it for some weeks now and wanna share my outcome.
Please don't be scared, I've rewritten a pretty huge part of the script to use btrfs subvolume set-default to mark the subvolume to boot.
I also tried to implement all your FIXME's.

I would appreciate it, if you find some time to have a look on it and tell me your thoughts.

Thanks,
@visit1985