Prefix configuration environment variables

[mrksmrtn]

The configuration package looks up keys from env vars in addition to what is loaded from the JSON config. This is convenient for overriding values with the environment. However, these names can conflict with existing system environment variables if they happen to have the same name. E.g.: A restlib config key named PATH would end up with the system path ($PATH), which is very unlikely what a user would want.

The solution is to automatically prefix the env vars which should be used with restlib. Something like: RESTLIB_PATH. This will greatly reduce the risk of a conflict and a hard to debug problem. Config keys would continue to have no prefix.