Add 'minify' option for all css/js/json output format transformers.

coffee.renderSync(str, {minify:true})

This will invoke second pass through uglify-{js;css;json} transformer.
The value can be minifier specific options as well, ie:

coffee.renderSync(str, {minify:{unsafe:true}})

will enable unsafe compression in uglifyjs2.

Signed-off-by: Karel Tuma <karel.tuma@gmail.com>