Add proxycommand support

This would fix issue #5. The first commit will add parsing of ~/.ssh/config if available. The second commit will use the parsed proxycommand. The last commit don't use the port from ~/.ssh/config since it defaults to 22 even when not present. I don't think that's something we want as usually the entry for ~/.ssh/config is for interactive use of a router. I put it in a separate commit if you think it should not be merged.