Cleaner command launch in shell_handler

[shellbertt]

1. Launch commands underneath an instance of bash to fully interpret &&, ;, etc.
2. Gracefully handle when the Command results in a panic.

Results:

• Can use cd in commands now without crashing (although directory will never persist between commands).
• Can do anything you can do in a line of bash opposed to one command and its arguments.
• A crashing command will not crash the handler.

Try:

SHELL 0 mkdir testdir
SHELL 0 cd testdir && pwd