neovim: adding python2 and python3 support

neovim:
- possibility to extend neovim (via .override) and passing extraPythonPackages
  or extraPython3Packages
- neovim's python interpreter can be found as nvim-python / nvim-python3
- wrapping nvim binary and setting `g:python_host_prog` and
  `g:python3_host_prog` via --cmd flag

python-packages.nix fixes:
- ordereddict builds for py26 and uses disabled argument to tell this
- trollius builds on all python platforms except 3.4 (where is included in
  standard librarary)
- neovim builds on all python platforms