Set the config via a JSON string

• Added ability to set configuration via JSON string instead of just a file

Usage of nitro-tftp:
  -c string
        Configuration file (shorthand)
  -config string
        Configuration file
  -config-string string
        Configuration string in JSON format
  -cs string
        Configuration string in JSON format (shorthand)
  -d    Enable debug logging (shorthand)
  -debug
        Enable debug logging
  -g    Generate example config (shorthand)
  -gencfg
        Generate example config
  -v    Display Version (shorthand)
  -version
        Display Version