Refactor logtarget - Githubissues

This PR refactors the logtarget package a bit, in order to make it more generic and useful in more situations, by making these changes:

LogTarget is now an interface
Changing the name of the default global variable to not "stutter" (logtarget.Target stutters)
Use io.MultiWriter to simplify the implementation
Allow the use of more than one writer as log outputs
Avoid hardcoding os.StdErr as the second output in the logtarget package - instead specify this in the server main function

mumble-voip / grumble