Add 'Fetch' command to nudb tool

[MarkusTeufelberger]

Might be useful for external tools/scripts (e.g. inserting data, retreiving data and asserting that it is the same).

[vinniefalco]

This is great, but it would be even better as a pull request!

[MarkusTeufelberger]

I can try, but I'm neither very deep into modern C++ nor boost at the moment, so don't expect anything great...

[vinniefalco]

No worries, lets just design what such a feature might look like. How about

nudb fetch <dat-file> <key-file> <key>

Where <key> is a hexadecimal string?

[MarkusTeufelberger]

Yeah, that's about what I would be expecting. The command would then print the data (probably also in Hex) or an error message.