fix: gitignore live config, add example template

Untrack config/derp.toml to avoid leaking credentials.
Ship config/derp.toml.example as the reference template.

config/derp.toml.example

@@ -0,0 +1,16 @@
+[server]
+host = "irc.libera.chat"
+port = 6697
+tls = true
+nick = "derp"
+user = "derp"
+realname = "derp IRC bot"
+password = ""
+
+[bot]
+prefix = "!"
+channels = ["#test"]
+plugins_dir = "plugins"
+
+[logging]
+level = "info"