Add middle-ticks between year and decade

[andrewcsmith]

The 10-fold increase between year and magnitude for the automatic setting of the Time axis is particularly difficult for narrow charts. It would be great to have the timeUnit automatically set the resolution at a finer grain, such as 2 years, 5 years, etc. (It could also be possible, once we get to years, to infer the ideal distance between ticks and use that.)

https://github.com/shutterstock/rickshaw/blob/master/src/js/Rickshaw.Fixtures.Time.js#L13