Make `ssh2` an optional peer dependency

[mark-dr]

Currently, DockerClient can't be used without ssh2 (a very large dependency) being in the dependency tree, despite it only being used in one branch of DockerClient.fromDockerHost. If consumers don't connect over ssh, they shouldn't need that dependency.

I'd recommend:

• Moving the static members out of DockerClient and into their own modules. That should solve the dependency tree problem (via tree-shaking) for anybody bundling their app
• For everyone else (and just to minimise node_modules impact), it'd be good to also make ssh2 an optional peer dependency and make the ssh://-specific code paths fail if it's not present

[ndeloof]

I'd be happy to make ssh2 an optional dependency, would you like to provide a PR for proposed solution ?

[mark-dr]

I don't have time at present, unfortunately. But at some point we'll be adopting this library and I'd be happy to look into it then.