Refactor flags and subcommands

[chclaus]

During the development process of dt the commands weren't designed consistently. An example:

The base64 command has four subcommands which defines the used algorithm. Each subcommand accepts a flag to determine if the value should be encoded or decoded. Furthermore there is a flag to pass a string that should be encoded or decoded.

In my eyes, a command or subcommand should do a task and the flags should be used to pass configure options for that tasks.

Given that idea, the base64 command works totally contrary to it.

In this task the whole set of commands and sets should be refactored to allow a cleaner API. That also means, that we have breaking changes and that will lead us to a new major version number. 🎉