Issue title: Get JSON config from URL
 Issue body: To ease integration with directory services and other configuration sources, `gnatsd` should accept a configuration URL that points to a JSON config.

example:
`gnatsd -c http://configserver.example.com:8181/nats?node=mynodename`

The config can be read using cURL or similar.
 Comments: 
 Comment 0: Agree, but this will not be all inclusive. Closing for now but this will be implemented, just not exactly like this.
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