Added an option to disable trigger snippets in Visual mode.

Hi Holger,
I've added an option to disable ``g:UltiSnipsExpandTrigger`` mapping in Visual mode. 

My use-case is simple, I use Tab to expand snippets, but in Visual mode I use Tab to fold selection. 
I don't want to change any of my key mapping, but as I don't usually work with snippets in Visual mode I just created an option to disable this mapping so I can use mine without any conflicts. 

Perhaps someone else will find this useful as well.