join: implement the -t option

tests/fixtures/join/semicolon_fields_1.txt

@@ -0,0 +1,6 @@
+1; a
+2 ;b
+3; c
+4 ;d
+5; e
+6 ;f

tests/fixtures/join/semicolon_fields_2.txt

@@ -0,0 +1,3 @@
+2 ;x
+3; y
+4 ;z

tests/fixtures/join/semicolon_separated.expected

@@ -0,0 +1,4 @@
+2 ;b;x
+3; c; y
+4 ;d;z
+

tests/test_join.rs

@@ -93,3 +93,34 @@ fn case_insensitive() {
         .arg("-i")
         .succeeds().stdout_only_fixture("case_insensitive.expected");
 }
+
+#[test]
+fn semicolon_separated() {
+    new_ucmd!()
+        .arg("semicolon_fields_1.txt")
+        .arg("semicolon_fields_2.txt")
+        .arg("-t")
+        .arg(";")
+        .succeeds().stdout_only_fixture("semicolon_separated.expected");
+}
+
+#[test]
+fn new_line_separated() {
+    new_ucmd!()
+        .arg("-")
+        .arg("fields_2.txt")
+        .arg("-t")
+        .arg("")
+        .pipe_in("1 a\n1 b\n8 h\n")
+        .succeeds().stdout_only("1 a\n8 h");
+}
+
+#[test]
+fn multitab_character() {
+    new_ucmd!()
+        .arg("semicolon_fields_1.txt")
+        .arg("semicolon_fields_2.txt")
+        .arg("-t")
+        .arg("э")
+        .fails().stderr_is("join: error: multi-character tab э");
+}