Create SASS/SCSS styles-outputter

[marcomontalbano]

With this outputter you can export all the styles as variables inside a .sass or .scss file.

This is a sample of the output:

$ tree output/
# output/
# └── _variables.scss

You can easily add this outputter to your .figmaexportrc.js:

module.exports = {
    commands: [
        ['styles', {
            fileId: 'RSzpKJcnb6uBRQ3rOfLIyUs5',
            outputters: [
                require('@figma-export/output-styles-as-sass')({
                    output: './output'
                })
            ]
        }],
    ]
}

Closes #59