add input file option

Allow feeding a bash-history-like file, which contains a list of timestamps.

After I saw @gak added a link to wordpress posting punchcard in README, I decided to finish what I had thought for long time, a [punchcard for blogger][bpunch]. Well, just some scripts pieced together to generate the images.

[bpunch]: http://blog.yjl.im/2013/06/blogger-posting-punchcard.html