New option `data` and more agile view template strategy.

- The option `data` can be accepts addition data to generating view.
  e.g. `-d="foo=foo,bar=bar"`.
- New view generating strategy. Default view template will be
  `view.txt`. For example, `php artisan generator:make posts.create`,
  when `views/create.txt` exists, use it (insead `view.txt`).