Add log_level option

Adding `log_level` option to plugin configuration.
It enables setting log_level separeted from global log level.

```conf
<source>
  type tail
  ...
  log_level error # can prevent "pattern not match"
</source>
```

This doesn't break existence plugins because a plugin can still use `$log` and `Input` /  `Output` use `$log` object in `log` method by default.
If plugin auther tries to support log_level, then needed action is only upgrading plugin's minor version with latest fluentd dependency.

Need some tests with actual configuration but tests now are passed.