add supports for the year optional field the same as in Quartz

[longzhiri]

Hello! I want to add the year optional field just like in Quartz. As the doc says the year field is range from 1970 to 2099,I need more bits to represent all years instead of a uint64. I use math/big.Int, because it's simple and I don't think would cause any performance issues.

[tomqwpl]

I currently also would like this. Having a year means you can express an individual point in time.

[tomqwpl]

@robfig Any thoughts on merging this?

[bamboo12366]

@robfig Any update regarding this PR?