Support running external commands

[jvalkeal]

This is an epic issue to add support for external commands. In a nutshell it'd be nice to create other spring-shell applications and run those via main shell instance. Take this as a basis of a plugin/extension system.

Until we get to an actual work two things will need some experiments. Firstly need to look if JetBrains pty4j can be used to implement pty features so that a process launched from spring-shell works as a proper shell. Secondly spring-shell core needs to be able to stop processing incoming events and pass those into a forked process.