Add option to preserve/enforce string quote style

`-q 0` (default) use single or double quotes such as to minimize the number of
 bytes (prefers double quotes when both will do); this is the previous
 behavior.

`-q 1` -- always use single quotes

`-q 2` -- always use double quotes

`-q 3` or just `-q` -- always use the original quotes.

Related codegen option: `quote_style`.

Close #495
Close #460

Some `yargs` guru please tell me why `uglifyjs --help` doesn't display the
help string for `-q` / `--quotes`, and why it doesn't output the expected
argument types anymore, like good old `optimist` did.

README.md

@@ -352,6 +352,13 @@ can pass additional arguments that control the code output:
   it will be prepended to the output literally.  The source map will
   adjust for this text.  Can be used to insert a comment containing
   licensing information, for example.
+- `quote_style` (default `0`) -- preferred quote style for strings (affects
+  quoted property names and directives as well):
+  - `0` -- prefers double quotes, switches to single quotes when there are
+    more double quotes in the string itself.
+  - `1` -- always use single quotes
+  - `2` -- always use double quotes
+  - `3` -- always use the original quotes
 
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