Add option to complete scrolled off words

[wellle]

As mentioned in #11.

Add option to complete words that are scrolled off in buffers.

Open questions:

1. For all panes or only the panes of the current window/session?
2. Settable number of lines into scrollback or all lines available?

@justinmk: Feel free to weigh in.

[wellle]

The command args introduced in #18 allow settings like this:

let g:tmux_complete_capture_args = '-J -S -1024'

(Join wrapped words and capture up to 1024 lines of scrollback)

See #18 (comment) for details.