Changes `lookup` to use `cmd` rather than `cmds`
Attaches `cmd.shell` within `Shell.prototype.process` and binds the `cmd` instance to `this` within builtins instead of passing arguments
WIP exposes builtin functions as commands
Removes `proc.write` and `proc.error` in favor of `proc.stdout.write` and `proc.stderr.write`
Refactor Shell.prototype.pipeline