allow config files to be independent from worktree location

[tazjin]

This is useful in cases where a config file is built by Nix (e.g. to ensure the availability of all formatters by templating their paths), as would be preferred for the TVL monorepo.

[zimbatm]

Can you also expand the path if the config-file is passed?

$ cargo run -- --config-file ./docs/treefmt.toml --tree-root .
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/treefmt --config-file ./docs/treefmt.toml --tree-root .`
thread 'main' panicked at 'assertion failed: treefmt_toml.is_absolute()', src/engine.rs:35:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[tazjin]

Can you also expand the path if the config-file is passed?

Done.