Refactor command-related code

[kylrs]

Description

This issue proposes the simplification of command-related code, and the implementation of a better, more object-oriented solution to command loading and parsing.

Why?

The current code for loading and parsing commands makes use of a number of "-er" classes, e.g. CommandLoader, CommandParser. Such classes tend to behave simply as collections of functionality, rather than definitions for proper objects.

Implementation Details

A single Command class that serves two functions:

• As a template for commands defined within modules, e.g.

class PingCommand extends Command {
    constructor(client) {
        super(client);
    }

    execute(message, args) {
        ping    
    }
}

• To contain static functions relating to commands in general, not just specific instances, e.g. a getCommand(msg: Message) method that identified a command by checking for a prefix and a valid command name, and returning the name of the command to be executed, if any.