Allow docker-gen to be used as a library

[md5]

As discussed briefly at #130 and nginx-proxy/nginx-proxy#254, it could be beneficial to restructure docker-gen to be usable both as a standalone binary and as an embedded library.

I'm opening this issue as a place to discuss the details of such a change.

[captncraig]

+1 I would like to make a plugin to another go app so it can reconfigure itself.

[captncraig]

I would love an api that notifies me anytime the set of running containers changes. Perhaps I can provide a callback that will receive []*RunningContainer anytime something new happens, or on an interval.

[md5]

FYI, I've started some work on this change here: https://github.com/appropriate/docker-gen/tree/generator

It's currently not compiling due to being in an intermediate state. I still need to figure out how to deal with the GeneratorConfig struct and I haven't had time to work on this in the last week.