Add body-output-dir option to the dir mode

[3niol4]

This PR introduces a new --body-output-dir option to the dir mode, allowing users to save the response body of matching paths to a specified folder.

Key points

• Add a --body-output-dir to the dir mode, taking a path as value.
• If the path doesn't exist it, create it
• If the option is set, save the response body of the requests to the matching paths to the specified folder. The name of each file is the path, with one modification: any / is replaced by _.