Add 'm' option - support for popup menu

Hello,
I made small update to your plugin.
The problem I met is that the ultisnips plugin breaks autocompletion when a snippet trigger is placed on the autocompletion list e.g:
- the same key triggers snippet expanding and completion (SuperTab plugin)
- there is a c snippet "printf"
- start autocompletion with the "prin" word, let say the following popup menu is shown:
   printf
   prints
   ....
- it is not possible to go through the completion list as the first item "printf" will trigger snippet expanding 

My proposal is to add new option - 'm'. Only snippets that define this option will be expanded when popup menu is visible.

Check if the patch is valuable.