Simple hack to allow tab in insert mode when completing function arguments

Although there has been a discussion and the issue was basically closed as WONTFIX I wanted to propose my simple hack for inclusion. It allows cycling through completable function/template arguments via \<tab\> in insert mode. When there is nothing to complete in the current line, \<tab\> works as usual. When the cursor is on a ')' or '>', i.e. after the last completion, a 'A' is inserted to go to the end of the line.