Allow the user to specify the used target directory manually

[xFrednet]

cargo-marker requires a target directory to fetch lint crates and to compile them. Usually, it extracts the default target location from the cargo metadata command. However, a user might want to manually specify it instead.

I believe the best solution for this, would be to add a new field to marker's configuration under the workspace.metadata.marker section in Cargo.toml

[workspace.metadata.marker]
target_dir = "./target"

[xFrednet]

This issue might depend on the discussion in: #251