App config handling with JSON/YAML de/encoder and builder

[arcticicestudio]

Epic: #33
Depends on #58

snowsaw will be configurable through JSON and/or YAML files in order to define general application-wide settings like e.g. the logging level.

Therefore the pkg/config package will provide

• a encoder interface with implementation to de/encode JSON and YAML configuration files
• a struct for general snowsaw settings
• project specific configuration constants

[arcticicestudio]

Resolved in #63 (5aa483e)