Fix #1413 Can't use bat at all! (Error: Use of bat as a pager is disallowed...)

Fixed by implementing the proposal by sharkdp:

* Allow PAGER=bat, but ignore the setting in bat and simply default to
  less. Unless of course, BAT_PAGER or --pager is used to overwrite the
  value of PAGER.

* Disallow the usage of bat within BAT_PAGER and --pager.

tests/integration_tests.rs

@@ -499,6 +499,28 @@ fn pager_disable() {
         .stdout(predicate::eq("hello world\n").normalize());
 }
 
+#[test]
+fn env_var_pager_value_bat() {
+    bat()
+        .env("PAGER", "bat")
+        .arg("--paging=always")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stdout(predicate::eq("hello world\n").normalize());
+}
+
+#[test]
+fn env_var_bat_pager_value_bat() {
+    bat()
+        .env("BAT_PAGER", "bat")
+        .arg("--paging=always")
+        .arg("test.txt")
+        .assert()
+        .failure()
+        .stderr(predicate::str::contains("bat as a pager is disallowed"));
+}
+
 #[test]
 fn pager_value_bat() {
     bat()
@@ -506,7 +528,8 @@ fn pager_value_bat() {
         .arg("--paging=always")
         .arg("test.txt")
         .assert()
-        .failure();
+        .failure()
+        .stderr(predicate::str::contains("bat as a pager is disallowed"));
 }
 
 /// We shall use less instead of most if PAGER is used since PAGER