add `sss` extension

[Krasnopir]

Can you add .sss type Sugarss is custom syntax for PostCSS and many projects are switching to using PostCSS, so it would be wonderful to be able to import the right files.

[mixtur]

Target languages support is depending on https://github.com/twolfson/spritesheet-templates. So it first should be supported there. Another option would be for you to write custom template.

[Krasnopir]

target: {
    ....
    css: [
        [
            path.resolve(__dirname, `/sources/sugarss/sprite.sss`),
            { format: 'sugarss_handlebars' }
        ]
    ]
},
customTemplates: {
    'sugarss_handlebars': path.resolve(__dirname, '/sources/sugarss/pngSpriteTemplate.handlebars')
},

It works!) Fine!)