Provide a command line interface

Hello,

Do you plan to also offer a command line interface of your library?

One of a the benefits of seeing "snappy" command line developed, would be the multithreading advantage. For example, when we use a "compressed" dd command, like this:

`dd if=/dev/sda status=progress | snappy > image.img.snappy`

Without having tried it, @nabijaczleweli proposes an [implementation](https://git.sr.ht/~nabijaczleweli/snappy-tools).

Thank you!