Download/install should support multi-binary installations

[kke]

Something like this should be possible:

$ pharos install 1.1.6
Installed pharos-cluster 1.1.6
Installed kubeadm 1.10.7
$ pharos use 1.1.6
$ pharos kubeadm ....

(Distributing / linking to other binaries may be problematic in license sense)

[kke]

Another option would be that the tool creates and handles symlinks/shims that point to/run correct binaries, like tools such as rbenv do:

$ pharos use 1.5.0
$ kubeadm --version
1.10.6
$ pharos-cluster --version
1.5.0
$
$ pharos use 1.1.0
$ kubeadm --version
1.10.2
$ pharos-cluster --version
1.1.0

This would however require something quite a lot fancier than what it does now.

[kke]

Implemented in #10