introduce assignments (#3345)

README.md

@@ -605,6 +605,8 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 - `arguments` (default: `true`) -- replace `arguments[index]` with function
   parameter name whenever possible.
 
+- `assignments` (default: `true`) -- apply optimizations to assignment expressions.
+
 - `booleans` (default: `true`) -- various optimizations for boolean context,
   for example `!!a ? b : c → a ? b : c`