Review result/overall.go

[martialblog]

We should review result/overall.go for the 1.0.0 release.

1. There's a note that we should deprecate the Outputs attribute:

    Outputs            []string // Deprecate this in a future version

2. The docs say that "Overall is a singleton for a monitoring plugin"

Do we need to mention that it is not concurrency-safe? Do we want it to be concurrency-safe?

3. Review the public methods and attributes and see if we can maybe hide some implementation details