Change module interface to conform common style

I see it's common pattern among other gulp plugins to do something,

```js
var diff = require('gulp-diff'):

gulp.task('smth', function () {
  gult.src('/a').pipe(diff('/b)).pipe(diff.reporter());
});
```

instead of

```js
var diff = require('gulp-diff').diff:
var reported = require('gulp-diff').reporter:

gulp.task('smth', function () {
  gult.src('/a').pipe(diff('/b')).pipe(reporter());
});
```

So, I've changed the interface of module a bit to conform that style :)