feat: add connection filters; and config struct;

also move sanitize_addr to mc_server module

README.md

@@ -1,2 +1,6 @@
 # Env variables:
 - BIND_ADDR(default: 0.0.0.0:25565): the address the server should bind to
+- FILTER_CONN(default: '(addr == "10.100.0.1")'): the filter appplied with [evalexpr](https://docs.rs/evalexpr/latest/evalexpr/)
+  The "context" has the addr variable populated with the address of part of the
+  handshake packet. Custom filter can be specified, and drop any
+  connections which match the filter.