Neovim: No Unicode decoding for replace_termcodes

- In Neovim the replace_termcodes will return invalid strings (according
  to &encoding). When calling this function we need to skip the
  automatic DecodeHook.
- The communicator now keeps two instances of the Nvim object, one with
  and one without the DecodeHook.
- Renamed Communicator.conn as .nvim

Fixes #76 for me, the following test previously failed with a decode exception
```
  :let g:weirdpath = 'weird¬p…l✓u↓g⏎i‽n'<CR>
```
What's the latest on getting this configured properly for `encoding`? I'm not terribly concerned with non-UTF8 setups and just left it at the default to begin with.