feat: add { flat: true } opt in the programmatic API

[jhack-jos]

Solves issue #86. { flat: true } is an alias of { up: true }.

Before this change you had to call: copyfiles( [source, output], { up: true }, callback )

Now the programmatic API mirrors the CLI and it is also possibile to call: copyfiles( [source, output], { flat: true }, callback )