introduce --safari10 (#2530)

README.md

@@ -111,6 +111,11 @@ a double dash to prevent input files being used as option arguments:
     --keep-fnames               Do not mangle/drop function names.  Useful for
                                 code relying on Function.prototype.name.
     --name-cache <file>         File to hold mangled name mappings.
+    --safari10                  Support non-standard Safari 10/11.
+                                Equivalent to setting `safari10: true` in `minify()`
+                                for `mangle` and `output` options.
+                                By default `uglify-es` will not work around
+                                Safari 10/11 bugs.
     --self                      Build UglifyJS as a library (implies --wrap UglifyJS)
     --source-map [options]      Enable source map/specify source map options:
                                 `base`  Path to compute relative paths from input files.