[PDF preview] Add PDF output option

[mgreau]

Add an option to choose PDF as a render output and show the PDF rendered in the frame on the "output side'.

Since AsciiDoctor use fopdf and it's based on docbook format, wWSmDA have to find a Java API in order to render PDF from HTML5 source.
2 API can be tested :

• FlyingSauce https://github.com/flyingsaucerproject/
• PDFBox http://pdfbox.apache.org/