Allow for specifing the target filename directly.

Thinking twice about my previous PR, more often than not I find myself linking a single file whose name I could specify directly instead of using a function call to return a plain string. This PR aims at fixing this, allowing for both

```js
gulp.src('somefile.ln').pipe(symlink('/tmp', '.somefile.ln'));
```

and

```js
gulp.src('*.ln').pipe(symlink('/tmp', function (name) {
    return name.replace(/(.*)\.ln/, '.$1');
}));
```

Tests and `README` have been updating accordingly.