Revert "Remove promotion to raw TCP based on heuristics"

This reverts commit fbaade4298 for the following reasons:

 - The commit only removed the proxy logic, while keeping the corresponding command line
   options etc. intact. That is quite confusing.
 - The switch is (and has been) off-by-default and the option help now clearly states
   that this needs to be used with caution. I'd argue that constrains the potential danger.
 - I have a specific use case that needs this, and implementing it as an addon is rather
   difficult at the moment.

That being said, this revert is a rather pragmatic temporary decision,
the functionality should clearly be made more explicit and protocol switching should
be moved to an addon.

mitmproxy/proxy/root_context.py

@@ -104,7 +104,16 @@ class RootContext:
             if alpn == b'http/1.1':
                 return protocol.Http1Layer(top_layer, http.HTTPMode.transparent)
 
-        # 6. Assume HTTP1 by default
+        # 6. Check for raw tcp mode
+        is_ascii = (
+            len(d) == 3 and
+            # expect A-Za-z
+            all(65 <= x <= 90 or 97 <= x <= 122 for x in d)
+        )
+        if self.config.options.rawtcp and not is_ascii:
+            return protocol.RawTCPLayer(top_layer)
+
+        # 7. Assume HTTP1 by default
         return protocol.Http1Layer(top_layer, http.HTTPMode.transparent)
 
     def log(self, msg, level, subs=()):