[Feature request] Completely customisable messages in Discord

[ashhhleyyy]

Similar to how DiscordSRV allows just about everything to be configured for the messages sent to Discord, BlockBot could support something similar.

My idea would be something like this:
Each message corresponds to some form of template file in the configuration directory (eg. config/blockbot-discord/messages/player_join.toml) that allows placeholders to be used (like the current beta config does) but also supports things like changing whether an embed is used or the username and avatar url of the webhook.

I'd be happy to implement something like this and open a pull request, but I thought it would be better to open an issue for discussion first.