Update colors.d.ts to work with TypeScript import

The file lacked a module delcaration, so it would not work with the following code:

    import colors = require("colors");

The `interface` does not belong in the `module` as it's global (applying to `String` and not scoped.)
Merge pull request #2056 from wiredprairie/patch-5

Update colors.d.ts to work with TypeScript import