Support for providing units with progress #172

In yarnpkg/lets-dev-demo#3 (comment) and yarnpkg/lets-dev-demo#3 (comment), I discovered that a project could reasonably want to write :elapseds to get (number of seconds)+s.

I spend a lot of time working w/ progress bars (I've worked on them in Mozilla, VirtualBox, Mercurial, and a number of other projects).

I can understand wanting to offer a time unit, it'd be nice if the node-progress API allowed for :elapsed_time or something which automatically provided a time unit. For something >70 seconds, it's probably better to list minutes, and for something >70 minutes, it's probably better to list hours, etc.