new option: --lib <path>

For @caridy - this option specifies a mojito path other than <cwd>/node_modules/mojito

It works for apps in the mojito/examples/ dir, but I do not want to support this option for general use, because:

1. only needed while old mojito installs an executable named "mojito" in your $PATH
2. only save the effort of symlinking old mojito to <app>/node_modules/mojito
3. app devs should just `npm i`

Examples:

```
cd path/to/my/app
mojito test app . --lib ~/Repos/mojito
mojito start --lib ~/Repos/mojito
mojito start -m ~/Repos/mojito     # -m for short
mojito jslint app . --lib ~/Repos/mojito
```