Add option to preserve/enforce string quote style

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.