Add <system> section to set core parameters in configuration file

For easy to deploy and setting. See this comment https://github.com/fluent/fluentd/pull/347#issuecomment-46038879

Example:

```conf
<system>
  log_level error # same as -q option
</system>
```

Remaining discussions:
- Configuration name, use `<system>` for now.
- What command line options should be set? Supported lists:
  - log_level
  - suppress_repeated_stacktrace
  - emit_error_log_interval
  - suppress_config_dump

What do you think?