enhance global_defs

- support arrays, objects & AST_Node
- support `"a.b":1` on both cli & API
- emit warning if variable is modified
- override top-level variables

fixes #1416
closes #1198
closes #1469

README.md

@@ -454,6 +454,8 @@ if (DEBUG) {
 }
 ```
 
+You can specify nested constants in the form of `--define env.DEBUG=false`.
+
 UglifyJS will warn about the condition being always false and about dropping
 unreachable code; for now there is no option to turn off only this specific
 warning, you can pass `warnings=false` to turn off *all* warnings.