Added a 'show post's author' option to widget.

### What?
It displays the post's/page's author that links to their profile page.

### Why?
I really enjoyed the multitude of options the 'Ultimate Posts' widget gave me. However, if a blog has more than one author, I want to give the admin the ability to display a post's/page's author with a link to their profile page.

### How?
I simply added a 'show_author' option to the widget's $instance array. I also added in a simple checkbox to the widget's form template.