Make the go program easier to use as a standalone program: make it a go module, add command line flags and support for ddnskey

[Golit]

This pull request will make the go program easier to use as a standalone program.

• Make the go source a go module since it is easier to use if used independently from the Dockerfile.
• The pull request removes the necessity of having a config file. All configurations can be passed with command line arguments.
• Some duplicate code is removed and refactored. Especially all copies of RequestDataExtractor where moved to a single file.
• Middleware architecture is used to gather user input variables which should make it more flexible.
• Changed go compiler to 1.14.

nsupdate improvements

• Commands to nsupdate are now piped to the program.
• A ddnskey can be passed from the http request to the nsupdate program.
• All commands send to the nsupdate program can be logged.
• User input values are escaped before passing them to the nsupdate program.

[dstapp]

Hey @Golit thanks for that huge contribution. I'll have a look at it as soon as I find some time for that!