Add functionality to use bindfs mounts instead of symlinks

[Freddo3000]

For some usecases such as #21 being able to use a bind or bindfs mounts would be more suitable. I suggest being able to switch out the first column with b to indicate a bindfs mount, as well as introducing key=value parameters such as bindfs="<bindfs args>" with the ability to be templated such as in FILELAYOUT.

It could also possibly help if you access your files on a system where unburden-home-dir isn't installed, as you would avoid lingering symlinks.