[Feature] Get command

[Artuto]

Pull Request

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Pull Request Information

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• [ ] My PR fixes a bug, error, or other issue with the library's codebase.
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Description

Adds the getCommand(String) method to the command client for retrieving a command with ease.

[jagrosh]

In my opinion, if this method is going to be separated, it should also be used internally when finding a command.

[kantenkugel]

This might clash with #71 which wasn't merged yet