Rewrite copy-to as Readable

[Panu Pitkamaki]

Extracting this from PR #103

In the future, we might want to extend copy-to out of Readable and not Transform which will avoid giving users duplex streams when the reality is copy-to = readable, copy-from = writable.

The copy-from side is now a Writable (#102). For symmetry, it'd be good if copy-to was a Readable (it's currently a Transform stream.)

[jeromew]

a PR for this can be found on #105

[jeromew]

in version 5.0.0 copy-to extends Readable

[jeromew]

in version 5.0.0 copy-to extends Readable