Add exclude patterns

I would love to see support for exclude patterns so that certain files and directories can be excluded from snapshots.

Currently I use nested subvolumes to exclude certain subdirectories in my home directory where only build artefacts etc end up. Since only the top level subvolume gets snapshotted that means those subdirectories/volumes effectively are left out of the snapshots.

However that has some drawbacks:
- I need to explicitly convert every directory I want to exclude to a subvolume, so I can't e.g. simply exclude all build directories with a single $HOME/**/build-out pattern.
- It's not possible to exclude files, e.g. the $HOME/.xsession-errors log (which happens to grow annoyingly quickly for me).

I think exclude patterns could work by taking writable snapshots, delete all files and dirs in them that match the patterns, and then change them to read-only.

As a simpler fallback to this feature, I'd like to see a btrbk config option that makes it create writable snapshots. Then I could just run my own script to regularly delete the stuff I don't want from them.

Thanks!