Add verbose flag for debugging purposes

[hermansc]

There's a lot of stuff happening in the background - requests being sent and services created. However, much of it happens without user knowledge and developers wanting to understand why the registrator is not working as expected are forced to add log.Println statements all over.

Instead, I propose a library such as jcelliott/lumber or /alexcesaro/log/stdlog for making logging at different levels.

Using log.Debug() and optionally log.Trace() statements we can even have --verbose=1 and --verbose=2. The "lumber" framework also supports adding prefixes, which I see is done in many of the log statements already (with "registrator: " as the prefix).

[progrium]

Sounds good. I'll look into those.

[hermansc]

After looking at Docker issues I found that they switched to using "logrus" couple of months back:

moby/moby#8761

Logrus: https://github.com/Sirupsen/logrus

[progrium]

Is this what you recommend?

[bketelsen]

+1 for logrus