add validation

#49

This adds `jsfmt --validate ./file.js`

```bash
Usage:
  jsfmt [--no-format] [--diff|--list|--write] [--validate] [--rewrite PATTERN|--search PATTERN] [<file>...]
  jsfmt (--version | --help)

Options:
  -h --help                      Show this help text
  -v --version                   Show jsfmt version
  -d --diff                      Show diff against original file
  -l --list                      List the files which differ from jsfmt output
  --validate                     Validate the input file(s)
  --no-format                    Do not format the input file(s)
  -w --write                     Overwrite the original file with jsfmt output
  -r=PATTERN --rewrite PATTERN   Rewrite rule (e.g., 'a.slice(b, len(a) -> a.slice(b)')
  -s=PATTERN --search PATTERN    Search rule (e.g., 'a.slice')
```

I dont really like not having a short version of `--validate` since we have them for all the others. Was thinking of just removing the short version of `--version` but open to other suggestions.