Since Vim version 7.4.849 [1] we can move in insert mode without
breaking the undo sequence by inserting <C-G>U before the movement key.
Add s:joinUndo which returns "\<C-G>U" if available.
Use s:joinUndo before doing any <Left> or <Right> movements in insert
mode to keep a single undo point which can be repeated and undone
with a single command.
[1] https://github.com/vim/vim/releases/tag/v7.4.849
