Add option to set manually wkhtml2pdf output file

For exemple :

```ruby
html = '<h1>test</h1>'
ouput_file = File.new(File.join(Rails.root, 'public', 'export-2013.pdf'), 'w+')
WickedPdf.new.pdf_from_string(html, output_file: output_file))
```

Might be useful in case you run this script from a web server that will get kill after ex. 10 sec. timeout. With this option set, the PDF file will get written where you want directly from the command line without using an other temporary file via Ruby (that might get kill if it is too long to generate).

I renamed the variable `output_file` as this is the option name in wkhtml2pdf documentation.