Introduce a filtered flow writer, and use it in dump.py

Fixes #104

doc-src/scripting/inlinescripts.html

@@ -13,7 +13,7 @@ lets the script interact with the global mitmproxy state. The __response__
 event also gets an instance of Flow, which we can use to manipulate the
 response itself.
 
-We can now run this script as follows:
+We can now run this script using mitmdump or mitmproxy as follows:
 
 <pre class="terminal">
 > mitmdump -s add_header.py