Add method for purging all post terms

*This Pull Request is the same as WebSharks/Quick-Cache#110--which is now closed--but I'm resubmitting this from my own fork so that there is some consistency to how we both contribute to the project. I will still retain responsibility for merging Pull Requests after they've been reviewed.*

---

@JasWSInc As per our discussions elsewhere regarding how this code should change, I'm submitting this new pull request for review. (I keep forgetting to keep such discussion on the Quick Cache repo so that it's public.) Please let me know your thoughts.

---

@JasWSInc wrote...
> @raamdev This looks great! The code looks pretty amazing from what I see.
> I'll test it shortly, but this looks ready to merge in to me.