Big cleanup of main.rs plus --exec

[nicklan]

This has a pretty big changeset:

• Create a CommandProcessor which handles all the processing of input, moved all that code from main.rs into command_processor.rs
• Move Env into its own file, env.rs
• Add completion to help
• Add unit tests for command_processor and env
• Make the completer not need unsafe anymore
• Support a --exec arg to exec a command
• Other small fixes + improvements