Modify tasks to use process.env.NODE_ENV convention

[gijsroge]

This would allows user to use the context in for example a PostCSS config (#19).

module.exports = (ctx) => {
    return {
        plugins: [
            require('rfs')(),
            require('postcss-inline-svg')(),
            require('autoprefixer')(),
            ctx.env === 'production' ? require('cssnano')() : false,
        ]
    }
};

Thoughts?

[MartijnCuppens]

Yup, would be better indeed. Could you resolve the conflicts here?

[gijsroge]

@MartijnCuppens done.