Add --relative-dates command line flag

Create --relative-dates command line flag which causes relative dates to be shown in all places when the flag is passed, except for CSV output, and places where an absolute and relative data were already shown together. Fixes issue #136.

I'll admit being motivated by the http://tip4commit.com/, but I thought solving an existing issue would still be helpful. Very nice project to hack on BTW; only took me 40 minutes, so thanks.