add whitespace delimiter option

tests/by-util/test_cut.rs

@@ -81,6 +81,16 @@ fn test_field_sequence() {
     }
 }
 
+#[test]
+fn test_whitespace_delimited() {
+    for param in ["-w"] {
+        new_ucmd!()
+            .args(&[param, "-f", COMPLEX_SEQUENCE.sequence, INPUT])
+            .succeeds()
+            .stdout_only_fixture("whitespace_delimited.expected");
+    }
+}
+
 #[test]
 fn test_specify_delimiter() {
     for param in ["-d", "--delimiter", "--del"] {

tests/fixtures/cut/whitespace_delimited.expected

@@ -0,0 +1,5 @@
+foo:bar:baz:qux:quux
+one:two:three:four:five:six:seven
+alpha:beta:gamma:delta:epsilon:zeta:eta:theta:iota:kappa:lambda:mu
+the	quick	fox	over	the	dog
+sally	sells	down	the	seashore	are	the	seashells	sally	sells