OutputStream: Added support for quote_char option

Can be either `'`, `"`, or `null` (`null` means choose the one with fewest occurrences in the string, like the previous behavior).

Use case: Serializing `data-bind` attributes (Knockout.js) into doublequote delimited HTML attributes while avoiding an excessive number of &quot; entities.

Corresponding pull req for UglifyJS1: https://github.com/mishoo/UglifyJS/pull/454 -- I've maintained a fork (`uglify-js-papandreou`) for some time because I need it for https://github.com/One-com/assetgraph