duration-values

Add support to decode `time.Duration` values from the environment.
This is pretty ugly, but I didn't find a better way to do this.  I
considered using `f.String()` which yields a value like
`<time.Duration Value>`.  That might be a bit more explicit/cleaner
than switching on the package path.  Either way, neither option seems
great.  Hopefully there's a better way that I'm just not aware of.