Avoid collision with JSON-LD

Since Logstash is using `@` as a prefix for reserved fields (like `@metadata`), I'd just like to point out that [JSON-LD](http://json-ld.org/) does the same and that since JSON-LD is a ratified W3C standard, it would be nice if Logstash did its utmost to avoid using the same field names to avoid collision.

The list of existing [keywords in JSON-LD](http://www.w3.org/TR/json-ld/#syntax-tokens-and-keywords) are:
- `@context`
- `@id`
- `@value`
- `@language`
- `@type`
- `@container`
- `@list`
- `@set`
- `@reverse`
- `@index`
- `@base`
- `@vocab`
- `@graph`