Expose words via tmuxcomplete#words

Adds a function `tmuxcomplete#words` that takes a single `scrollback` parameter. A scrollback of `0` will return only visible words. A scrollback of `100` will consider 100 scrolled off lines in each pane.

Here are some numbers for my current tmux sessions:
```vim
echo len(tmuxcomplete#words(0))    # 1348
echo len(tmuxcomplete#words(10))   # 2077
echo len(tmuxcomplete#words(100))  # 7893
echo len(tmuxcomplete#words(1000)) # 32865
```

Close #12.