[PROPOSE] allow @ prefix in parameter name if double/single quoted

This is a proposal. On using fluent-plugin-record-reformer or fluent-plugin-rewrite-tag-filter, and Elasticsearch, users want to emit messages with `@timestamp` field as:

```
<match **>
  type record_reformer
  <record>
    @timestamp ${time}
  </record>
</match>
```

But, with --use-v1-config, @ prefix is reserved as a system parameter prefix, so we get errors as following:

```
'@' is reserved prefix. Don't use '@' in parameter name
```

This pull request is a proposal to allow users to set `@` parameter name if double/single quoted string is used such as:

```
"@timestamp" ${time}
'@timestamp' ${time}
```

cf. https://twitter.com/yoshi_ken/status/520501876492480513 (Japanese)
cf. https://twitter.com/sonots/status/520503664146788352 (Japanese)

Another idea to resolve this issue is welcome if you have.