Maximum popup menu width option

I think it would be useful to have an option to limit the maximum width of the popup menu, so the quick help tooltip can be shown even when completion summaries are very long. 

Here is a patch that adds such an option. The maximum width can be set both as an integer (fixed maximum width) or as a float between 0 and 1 (for a maximum width that depends on the window width). 

I also added another customization option to add a postfix to the summary when it gets truncated.

Hope you find this useful.