Add target fsautocomplete into Vim plugin Makefile
Add target fsautocomplete into Vim plugin Makefile

fsautocomplete is a fixed path-independent pseudonym for target which makes
fsautocomplete.exe. It might be useful when Vim plugin is installed (and
built) by one of Vim plugin managers (NeoBundle, vim-plug etc.).

fsharp/vim-fsharp,fsharp/vim-fsharp