Allow deadkeys that do not require a modifier key to work

[bambu]

Improved deadkey recognition. Now it should work on layouts where a modifier key is not required for deadkeys. This pr is for issue #221. I tested it with a greek layout and it seemed to work fine with the ;e being έ. I also tried other random letters and was able to get άίόύ.

[rogual]

Thank you!

[nrontsis]

@bambu Thank you for the PR! I was about to write about it. It works fine and it fixes #221! The only difference with MacVim is that when typing ; it goes into im insert mode indicating that a potential modifier key was typed (a similar thing happens when running inside a terminal); this does not affect the functionality in any way though.