Add support for command-line flags to configure application

[evanebb]

• Moves retrieving the application configuration outside of the Server struct into its own AppConfig struct for better separation/abstraction and for better 'documentation' as to what configuration is needed instead of it being scattered across a few os.Getenv calls.
• Changes most of the environment variables to include a GOBBLE_ prefix; this ensures that no conflicts between different applications that might use the same generic environment variable names will occur (when running outside of Docker for example).
• Adds support for configuration through a set of command-line flags. This is very handy for development on my local machine, where I prefer using these over environment variables.