Make logging pluggable about eventstore-client-go

[YoEight]

@marcinbudny Do you have a library that you like on this? I'm not a Gopher so I'm looking for examples.

[marcinbudny]

I'm currently using logrus, however I don't think adding a specific logger to ESDB client is the way to go. ESDB client is a library and it should enable its users to choose what they want to use.

If there was an abstraction layer for logging, I could implement a simple adapter for logrus. This abstraction layer could be something similar to the one proposed in this repo. This is just an example, instead of adding it as a dependency, ESDB client could define its own logging interface.

The default implementation could write to console, but then I could replace it with my implementation that writes to logrus.

BTW I'm not a Gopher myself, so maybe there is a more idiomatic way to do this 😉

[YoEight]

@marcinbudny if possible, I'd like to have your input on #119

The PR brings a pluggable logging implementation based on a function type.