Add custom command classes to generate processed output

[aydwi]

Instead of dumping a std::string of stream output to the client, Barrel can do some processing based on the exact brew command it executed and return, for example a neat std::unordered_map or other various (applicable) types.

This feature is not finalized because:

1. It violates the "one job" philosophy.
2. It comes with a subjective choice of C++ types and might force advanced clients to do ugly casting, which defeats the entire idea of doing it in the first place.
3. It cannot be fool-proof or comprehensive. brew does dynamic things with certain commands which cannot all be encapsulated before execution (at least with any reasonable effort).
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But it could still be useful for people who want a baked cake instead of the ingredients.

This feature will not be a part of the inital release, but will remain open for further thought till a decision is made.