Cover remaining events and endpoints, update to Discord v10

[lcsmuller]

What?

• Cover all of Discord API's event callbacks
• BSD Makefile compliant build system, enable -O2 compilation flag per the default
• Cover remaining endpoints
• Update to Discord v10

Why?

On-going effort to keep Concord up to date with the Discord official API

How?

Add discord_set_on_*() functions for remaining event callbacks, add function wraps for remaining endpoints and update DISCORD_API_BASE_URL and DISCORD_GATEWAY_URL_SUFFIX value to match Discord v10

Testing?

Added examples to cover newly added events and endpoints