Add negate_iife option to the code generator.

See discussion in a9511dfbe5

README.md

@@ -287,6 +287,10 @@ can pass additional arguments that control the code output:
   you pass `false` then whenever possible we will use a newline instead of a
   semicolon, leading to more readable output of uglified code (size before
   gzip could be smaller; size after gzip insignificantly larger).
+- `negate-iife` (default `!beautify`) -- prefer negation, rather than
+  parens, for "Immediately-Called Function Expressions".  This defaults to
+  `true` when beautification is off, and `false` if beautification is on;
+  pass it manually to force a value.
 
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