Not map <c-x> if already mapped to something else

I am using <c-x> as cut like every other editor. In insertion mode, this mapping will cut the word under cursor. Unfortunately your plugin will automatically remap <c-x> to execute ManualCompletionEnter(). I added some code to avoid this mapping is <c-x> is already user mapped. I also added a g:SuperTabMappingTabManual to let the user choose its own <c-x> mapping.