Drop by key feature

[therustmonk]

This library hasn't feature to drop cached value by key ( I can drop all keys by PersistentCache.clear only).

It's good to have clear with setting by key feature as well.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.