Fix: Decompress no longer defaults to overwrite the destination directory, and deleting files inside.

[joshrandall8478]

This aims to fix issue #813 with ouch prompting to overwrite the destination directory, in turn deleting files in the destination, despite no conflicts. This stems from a logic issue on line 324 in decompress.rs.

It now runs the following checks:

• If output_dir does not exist, it's valid and will be created
• If output_dir exists AND is a directory, it's valid and will be used (even if non-empty)
• If output_dir exists AND is a file, there is a conflict and will ask the user.

let is_valid_output_dir = !output_dir.exists() || output_dir.is_dir();

[joshrandall8478]

Some tests may need to be updated to match new behavior. I am open to contributions / suggestions.