Respect quote style in object literals

The option added in fbbaa42ee5 wasn't
being respected inside object literals, so quoted property names would
still be stripped out with this option.

This is mostly a corner-case, but useful when the output is passed to
something like the Closure compiler, where quoted property names can be
used to prevent mangling.

README.md

@@ -133,7 +133,16 @@ The available options are:
   --reserved-file               File containing reserved names
   --reserve-domprops            Make (most?) DOM properties reserved for
                                 --mangle-props
-  --mangle-props                Mangle property names
+  --mangle-props                Mangle property names (default `0`). Set to 
+                                `true` or `1` to mangle all property names. Set
+                                to `unquoted` or `2` to only mangle unquoted 
+                                property names. Mode `2` also enables the
+                                `keep_quoted_props` beautifier option to 
+                                preserve the quotes around property names and
+                                disables the `properties` compressor option to
+                                prevent rewriting quoted properties with dot
+                                notation. You can override these by setting
+                                them explicitly on the command line.
   --mangle-regex                Only mangle property names matching the regex
   --name-cache                  File to hold mangled names mappings
   --pure-funcs                  List of functions that can be safely removed if
@@ -479,6 +488,8 @@ can pass additional arguments that control the code output:
   - `1` -- always use single quotes
   - `2` -- always use double quotes
   - `3` -- always use the original quotes
+- `keep_quoted_props` (default `false`) -- when turned on, prevents stripping
+  quotes from property names in object literals.
 
 ### Keeping copyright notices or other comments
 
@@ -667,7 +678,8 @@ Other options:
 
 ##### mangleProperties options
 
- - `regex` — Pass a RegExp to only mangle certain names (maps to the `--mange-regex` CLI arguments option)
+ - `regex` — Pass a RegExp to only mangle certain names (maps to the `--mangle-regex` CLI arguments option)
+ - `ignore_quoted` – Only mangle unquoted property names (maps to the `--mangle-props 2` CLI arguments option)
 
 We could add more options to `UglifyJS.minify` — if you need additional
 functionality please suggest!