feat: add explain and loging features (#40)

* explain features

* feat(core): Add  feature

Cargo.toml

@@ -22,6 +22,8 @@ futures = "0.3"
 
 [features]
 default = ["runtime-tokio"]
+explain = ["casbin/explain"]
+logging = ["casbin/logging"] 
 
 runtime-tokio = ["casbin/runtime-tokio", "tokio/sync"]
 runtime-async-std = ["casbin/runtime-async-std", "async-std/std"]
@@ -37,4 +39,4 @@ lto           = true
 opt-level     = 3
 
 [profile.dev]
-split-debuginfo = "unpacked"
+split-debuginfo = "unpacked"