Feature/expose add font family

[Shayaomer]

In order to add custom font that loaded locally in my project, it's essential to insert the font name into the exisiting fonts dictionary. Thus I exposed the AddFontFamily method to the PdfGenerator class, in order to add my custom fonts to the _exisitingFont dicitonary and use those fonts instead of the default Segoe UI.

I havn't found another way to use custom fonts on non-windows machines.

Fixes #122