Add tmux command arg options

This adds options to modify the `tmux` commands.

Defaults:

`let g:tmux_complete_list_args = '-a'`
Additional args for `tmux list-panes`.

`let g:tmux_complete_capture_args = '-J'`
Additional args for `tmux capture-pane`.

Examples:

`let g:tmux_complete_list_args = '-s'`
Only capture panes of active session.

`let g:tmux_complete_list_args = ''`
Only capture panes of active window.

`let g:tmux_complete_capture_args = '-J -S -1024'`
Join wrapped words and capture up to 1024 lines of scrollback.