Add out_sql plugin

I implemented `out_sql` to insert events to RDBMS.
Configuration is similar to `in_sql`.

```conf
<match sql.**>
  type sql
  host 127.0.0.1
  port 3306
  database fluentd
  adapter mysql2

  # Optional. event which has sql.project.xxx tag is stored into this table
  <table sql.project.**>
    table projects
   # json keys to table columns. name to foo, age to bar.
    key_names name,age
    column_names foo,bar
  </table>

  # Default table. It is required.
  <table>
    table users
    column_names name,age # key_names is not needed if key_names and column_names are same.
  </table>
</match>
```

Currently, this is a prototype. Any feedbacks are welcome.