add --timeline option to draw virtical line every 10/60 seconds
I needed timeline for daily operation. "--timeline" option is useful for debugging and snapshots sharing. Thank you

Hi,
Thank you for the pr. First, it LGTM. However, ping timings for all targets are not the same, especially in not async mode (It can happen in async mode if there are too many targets). The attached screenshot presents this issue. So, I cannot merge this pr as is. But I understand your motivation. I will consider it.
BTW, printing timestamp on the 3rd line looks good. If you extract it (https://github.com/shigechika/deadman/commit/c3a3316c6e490d1687fc09e55c54706b38a69bc5#diff-4a173307c12a52d82e6ad896a4dedcfb3b98c2724f557579c73a309d49d04413R479-R482) and make a pr, I'd like to merge it soon.

--timeline option is usefull with --async-mode. I propose that --timeline option force to enable --async-mode. Please consider again. Thank you