Add an "unstable" feature to Cargo.toml

[sophacles]

There's a few features folks are wanting that are "unstable" in i3 - particularly Node attributes and the like. Since i3 changes quite slowly, these attrs can be around for years. It seems like they could be added and guarded by a feature.

Then it could be simply enabled by something like:

[Dependencies]
i3rpc = { features = ["i3-next", "unstable"] }
...

It also gives a nice path forward for when i3 does update its stable features to include them.

[sophacles]

This could be extended for sway and i3-gaps specific features.