Update `cpy` to v9

[andy2mrqz]

As pointed out in #32, glob-parent reports a high vulnerability through npm audit. Since this has been addressed in cpy v9.0.0, this PR updates to v9.

Brief description of changes:

• Update cpy to v9.0.0
• Update cli.js to use new --flat flag
• Add documentation examples; remove references to deprecated flags
• Add tests (two copied from cpy) and update to conform to v9

Please let me know if you'd prefer any changes to tests, documentation, or have any questions.

Fixes #32 Fixes #27