Accept '@type' and '@id' in v1 configuration. fixes #500

Plugin type and id are system reserved parameters like @label.
So using '@' prefix is better for separating normal plugin parameters.
in_monitor_agent supports '@type' and '@id' query
Merge pull request #503 from fluent/at-prefix-for-type-and-id-parameters

Accept '@type' and '@id' in v1 configuration. fixes #500