Custom method overrides for all available gateway codes

[n7st]

When building the class, overriding the default methods (inside Discord::Client) for opcodes from Discord should be allowed.

Presently, the client is created like:

my $gateway = Discord::Client->new({
    auth_token     => 'DISCORD AUTH TOKEN HERE',
    user_agent     => 'Some user agent string here',
    callback_class => $main_class,
    routines       => {
        each_message => 'message_received',
        finish       => 'finished',
        connect      => 'connected',
    },
});

routines should be adjusted to allow gateway codes or names to be passed in.