Add trailing whitespace segment

Note: by default this segment is disabled. Until #923 it may only be enabled by
copying the whole file and changing "enabled" to "true". After #923 it may be
enabled by having `~/.config/powerline/themes/vim/default.json` with the
following contents:

    {
        "segment_data": {
            "trailing_whitespace": {
                "enabled": true
            }
        }
    }

Fixes #388