Added the ability to explicitly calculate and cache a given range of holidays

Hi there,

Our app does a lot of calculations involving business dates, and we previously had a hard-coded list of holidays. After switching to the `holidays` gem, we noticed that some of our requests were taking 30% longer because of many holiday calculations.

This pull request adds a simple `Hollidays.cache_between` method, so that we can calculate and cache a list of holidays when the app boots. For example, I've added this in an initializer:

```
Holidays.cache_between Time.now, 2.years.from_now, :us, :observed
```

Please let me know if you think there's a better way to do this.