add rand as optional but default dep

Cargo.toml

@@ -26,11 +26,16 @@ num-rational = "0.2.3"
 num-bigint = "0.2.6"
 num-traits = "0.2.11"
 once_cell = "1.3.1"
+rand = { version = "0.7.3", optional = true }
 
 [features]
-default = ["commandline"]
+default = ["commandline", "random"]
+# Option (enabled by default): build a binary using clap
 commandline = ["clap"]
+# Option: enable nightly-only features (for right now, only the `track_caller` attribute) 
 nightly = []
+# Option (enabled by default): enable the builtin function `random([$limit])`
+random = ["rand"]
 
 [dev-dependencies]
 tempfile = "3"