Overrides doesn't match relative paths

JSHint doesn't override configuration when it is called with a relative path like `./src/bar.js` while the pattern is for `src/bar.js` for example (the opposite is also true).

```json
{
  "overrides": {
    "src/bar/js": { "asi": true }
  }
}
```

```
$ jshint src/bar.js     # => overrides configuration
$ jshint ./src/bar.js   # => won't override configuration
```

This patch tries to fix the problem by normalizing the file path, which transforms `./src/bar.js` into `src/bar.js` so that the overrides pattern will match both cases transparently. It's still compatible with relative patterns like `../src/bar.js`  since `path` won't normalize a leading `..` (but there is no test to assert it)