Documentation improvements

[kffl]

The set of functionalities offered by Gocannon has expanded quite considerably in the last couple of releases. There are some areas of the documentation that could use a bit of an improvement:

• Add a short guide on writing custom Gocannon plugins
• Describe each config flag/arg in more detail
• Add information about the available docker container images
• Document the JSON/YAML output schema for both request log and histogram modes
• Add info about using Gocannon as a library/dependency for programmatic load tests