Add cron parser for start time of a reminder

Cargo.toml

@@ -35,6 +35,7 @@ serenity = { version = "0.12", default-features = false, features = ["builder",
 oauth2 = "4"
 csv = "1.2"
 sd-notify = "0.4.1"
+cron-parser = "0.10"
 
 [dependencies.extract_derive]
 path = "extract_derive"