message: appender: reduce tracing-subscriber's required features to `fmt`
  Resolves
diff: @@ -20,10 +20,15 @@ keywords = ["logging", "tracing", "file-appender", "non-blocking-writer"]
  edition = "2018"
  [dependencies]
  -tracing-subscriber = {path = "../tracing-subscriber", version = "0.2.7"}
  crossbeam-channel = "0.4.2"
  chrono = "0.4.11"
  +[dependencies.tracing-subscriber]
  +path = "../tracing-subscriber"
  +version = "0.2.7"
  +default-features = false
  +features = ["fmt"]
  +
  [dev-dependencies]
  tracing = { path = "../tracing", version = "0.1" }
  tempdir = "0.3"
  