Add warning for level > 3 for non-zlib

It's not obvious immediately that these levels don't have any effect on libdeflater and Zopfli, since they don't iterate over zlib-specific fine-tuned options.

Hence, show warning so that user knows they're getting "downgraded" to level 3.

src/main.rs

@@ -321,16 +321,13 @@ fn parse_opts_into_struct(
         .init()
         .unwrap();
 
-    let mut opts = if let Some(x) = matches.value_of("optimization") {
+    let level = match matches.value_of("optimization") {
-        if let Ok(opt) = x.parse::<u8>() {
+        Some(x) => x.parse::<u8>().unwrap(),
-            Options::from_preset(opt)
+        None => 2,
-        } else {
-            unreachable!()
-        }
-    } else {
-        Options::default()
     };
 
+    let mut opts = Options::from_preset(level);
+
     if let Some(x) = matches.value_of("interlace") {
         opts.interlace = x.parse::<u8>().ok();
     }
@@ -494,6 +491,17 @@ fn parse_opts_into_struct(
         }
     }
 
+    if level > 3 {
+        match opts.deflate {
+            Deflaters::Zlib { .. } => {}
+            _ => {
+                warn!(
+                    "Level 4 and above are equivalent to level 3 for compressors other than zlib"
+                );
+            }
+        }
+    }
+
     if let Some(x) = matches.value_of("threads") {
         let threads = x.parse::<usize>().unwrap();