Snap support

[mhalano]

I'm developing a Snap for kubectx+kubens and I have a functional version. I will post a PR shortly, but some points still remains:

• It needs an interface connection, which isn't automatic. The idea is to open a request to enable the auto-connection for this interface
• It requires an alias. The Snap is called "kubectx", but it has two commands: kubectx and kubens. The first one works fine, just type kubectx and that is it, but the second one, however, requires more effort. You need to type kubectx.kubens. An alias can be requested to just type kubens, but again, a request has to be done
• A third question is we could try to use to tracks, one for go version and another one for bash version, but more research needs to be done about that
• I couldn't make the auto-completion work on my machine, but I use Zsh, not Bash, so help will be appreciated.