Support the '--watch' compiler flag.

Fixes #268.
move this above the check for file count, and have a sys.exit(0) as well..
```TypeScript
if (version) {
    // print version
    sys.exit(0);
}
if (help) {
    // print help
    sys.exit(0);
}
if (files.length === 0) {
    // print error
    // print help
    sys.exit(1);
}
```