vimux breaks some characters with diacretics

This is actually the same problem as with [vim-rsi](https://github.com/tpope/vim-rsi), see https://github.com/tpope/vim-rsi/issues/14.
tpope introduced an option to not map the META-key, however it would  be nice, if the META-keybindings could be used and characters with diacretics as well. Maybe you have an idea.
