Colors & style

It would be great if we had some colors in the output (to make distinction between additions and removals, for example). Even better if the style matched the output of `cargo update`.

Start by locating the code that formats cargo-depdiff's command-line output, then compare its additions and removals with the output of `cargo update`. Define which colors and style conventions should be matched, and consider the output complete when additions and removals are clearly distinguished with consistent formatting.

Written by the indexing model from the issue text.