Implement SSH client

[kirb]

This will be where we start to split into a jailbreak/Mac build and App Store build.

Likely want to use SwiftSH.

Will need to replace SubProcess with a protocol that can support both a local and remote shell.

Will also change iOS app structure to a split view at root with list of servers and sessions. Can probably implement this on macOS as well, but at least initially may be better to just show sessions in a window that then simply opens a new local tab and runs an ssh command line. i.e. no remote support built in.