Update postcss.d.ts for Input interface (#851)
Update postcss.d.ts for Input interface (#851)

Example
```typescript
import { plugin, Container, Result } from "postcss";
plugin("postcss-test", function() {
   return function(css: Container) {
      const input = css.source.input;
      console.log(input.css);       // haven't access to css property in typescript
                                                   // but valid in javascript
   };
});
```
postcss/postcss,jonathantneal/postcss,pascalduez/postcss,postcss/postcss,bezoerb/postcss,alanev/postcss,Semigradsky/postcss