Allow for basic manipulation of target filenames.

This PR aims at providing support for basic manipulation of target filenames. A simple use case is the following task:

```js
gulp.src('src/*.ln').pipe(symlink('dst', function (name) {
    return ('.' + name.replace('.ln', ''));
}));
```

Supposing that `src` is

```
src
|-- a.ln
|-- b.ln
|-- c.ln
```

then `dst` will become

```
src
|-- .a
|-- .b
|-- .c
```