Added FileMailer implementation of mailer interface

Added `yii\swiftmailer\FileMailer` that saves emails under runtime directory, which can be useful during development.

 `yii\swiftmailer\FileMailer` has 2 properties:

- `path` - the path under which email will be written. If it is not set, `@app/runtime/mail` directory will be used
- `callback` - the callback that is used to generate filename