make compatible with dpy 2.0 and migrate to slash commands

[LBlend]

Closes #11

All commands have been migrated to slash commands, the exception being the dev tools commands since they require to pass the is_owner check which is currently not suppported with the new slash command framework.

The bot as a whole has now, and will only work on 2.*.* releases of discord.py

At the same time I have changed the following:

• removed mlm command
• specified dependency major and minor versions in the requirements file
• added default values to the example config file

[marksverdhei]