modified cli.js so that it can read from piped input by default

I modified `cli.js` so that it expects stdin by default if no arguments are provided and will detect when no stdin is provided:

- `stdin` piping before:
```
$ cat package.json | js-beautify -f -
{{ pretty output }}
$ cat package.json | js-beautify
Must define at least one file.
Run `js-beautify.js -h` for help.
```

- `stdin` piping after:
```
$ cat package.json | js-beautify -f -
{{ pretty output }}
$ cat package.json | js-beautify
{{ pretty output }}
```

- before with no `stdin`:
```
$ js-beautify -f -
{{ hangs until ctrl+c }}
$ js-beautify
Must define at least one file.
Run `js-beautify.js -h` for help.
```

- after with no `stdin`:
```
$ js-beautify -f -
No input detected
Run `js-beautify.js -h` for help.
$ js-beautify
No input detected
Run `js-beautify.js -h` for help.
```