Followup #136; change loggerSetup to onLoaded

[nitely]

Followup #136

Changes:

• loggerSetup to onLoaded
• The onLoaded takes a config var parameter

[arnetheduck]

Come to think of it, an alternative way to implement this is via compile-time lookup, ie a mixin-based approach where users can provide onLoaded with a higher generic matching specificity, similar to how serialization finds readValue.

Not sure if it's worth the bother, but it's maybe more in line with the confutils architecture overall (same as with overloading parseCmdArg)

[nitely]

ah yes. But the onLoaded name is too generic. In nimbus-eth2 you would overload onLoaded(conf: var object), it should be named `onLoadedConf` or something else. Also, there would be no way to make it conditional, not sure if this matters.

[nitely]

In nimbus you would overload onLoaded(conf: var object)

mmh, I think nimbus could just expose a setup logger overload generator:

template setupLogger*(ConfType: type): untyped =
  proc onLoaded(conf: var ConfType) =
    # ... setup logger

then do setupLogger(MyConf) everywhere where MyConf is loaded.

or more generally:

template setupConf*(ConfType: type, setupLogger: bool): untyped =
  proc onLoaded(conf: var ConfType) =
    when setupLogger:
      # ... setup logger