Using Gocannon as a library

[kffl]

As of now, Gocannon can only be used as a standalone program (with optional support for custom plugins). After making some changes to the codebase (exposing exported methods and tying the config struct to a specific instance of Gocannon), it would be possible to use Gocannon as an external library called from other Go code (i.e. for the purpose of programmatically performing a load test).