Add option to specify global default layout path

I found out that there is no option to specify default layout. If layout was set to true, layout name was set to 'layout'. This fix add an option to specify default template in 'view options'.

Example usage:
app.set("view options", { defaultLayout: 'layouts/default'});
Merge pull request #42 from mateuszgachowski/patch-1

Add option to specify global default layout path