Use clap for argument parsing, and allow a --target argument to choose which target to download packages for

The cookbook could be used to build for multiple architectures, if that
is ever supported.

Define TARGET environment variable for Travis

Use BufReader to extract gzip archive

Use pbr crate for progress bar

Refactor with a Package class

Add PackageMeta struct

This reverts commit a0ec9c60.

Use tar-rs library directly, instead of calling tar binary.

Make executable support install from path

This can be handy to have.

Make it possible to use pkgutils library on Linux and install to custom destination

Probably it would be better to get this at runtime somehow... anyway, I
needed this change to be able to run it on Linux (to bootstrap a Redox
install)

For library; not the pkg utility yet