Add neocomplete integration

Add source for [neocomplete](https://github.com/Shougo/neocomplete.vim).

Add option to choose between:
```vim
let g:tmuxcomplete#trigger = 'completefunc' " <C-X><C-U> (default)
let g:tmuxcomplete#trigger = 'omnifunc'     " <C-X><C-O>
let g:tmuxcomplete#trigger = ''             " no trigger
```

Close #29.