Add support for running shell commands as sub-operations

[TheFriendlyCoder]

Sometimes it is helpful to be able to run shell commands from the parent console / shell under which the friendly shell is running. We should look to add support for exposing console tools as built-in commands for the API.

One use case I can see is if someone wanted to query the host system to, say, get an IP address or host name of a service to connect to it'd be nice to run the command directly in Friendly Shell to get the result from the console without having to exit from the friendly shell and re-launch it again afterwards ... or having to launch a second console / terminal instance to run the command and copy-pasting the output back into friendly shell.

[TheFriendlyCoder]

To maintain some compatibility / familiarity with the 'cmd' builtin module, we should make commands that start with an exclamation mark references to shell commands, spawning sub-processes to run the shell commands.

[TheFriendlyCoder]

To facilitate integration with the Friendly Shell IO subsystem we should use subprocesses to capture output produced by the shell command and redirect it to the appropriate output streams.