Python 3 support

[lbmn]

To get it working in Termux (which defaults to Python 3, as everything should), I had to:

• apt install vim-python

• ~/.vim/bundle/nim.vim/autoload/nim.vim: replace the "pyfile" line with:
  exe 'py3file ' . fnameescape(s:plugin_path) . '/nim_vim.py'

• ~/.vim/bundle/nim.vim/autoload/nim_vim.py: remove Queue from the import line, and add:
  from multiprocessing import Queue

[optik-aper]

@lbmn: Thanks for figuring that out.

Is there any plan to add python3 support upstream for this plugin?