Restore brightness correctly when using `--transition-time` with `on`/`off` command

Using a transition time to e.g. turn off a light results in a very dimmed light when turned on again. The `phue` package solves this by caching the brightness before turning off the light internally, this isn't persisted across invocations of the program however, so we'd need to implement a similar mechanism ourselves (and persist to a directory like `.config/lights`).

> Note that directly specifying the brightness will of course always work, e.g. `lights dim 100` instead of `lights on`