Filter commits by author date instead of commit date with -R flag

[hallzy]

The -R flag shows you the author dates, but the commits aren't filtered on author date (if you only want to see the commits with an author date since yesterday you can't do that, it will show you the commits with a commit date since yesterday, but with the author dates displayed).

I've added functionality to filter on author date when the -R flag is passed. Unfortunately, it seems that git log cannot do this natively, so the log command is a bit more complicated, but seems to work.

[wizpig64]

This fork is exactly what I needed out of git-standup. Thanks!