Hack to enable neovim support

[malramsay64]

This is a very first attempt at enabling neovim support for vim-taskwarrior #132 . In my use the only functions that require interaction are undo and delete, for these two functions there is a check has('nvim') which changes execution from ! to terminal and there is also no taskwarrior#refresh() call, which is done using a BufEnter autocmd.

There is likely a better solution that I don't yet know about.