add ability to evict the cached value

[ianstormtaylor]

Instead of clearing the entire cache, just being able to evict a single value by key. But handled under the covers such that it just adds an extra exict() argument to the render prop function's arguments.

Similar to #176

[jamesplease]

Thanks for the issue! I am open to adding something like this, although I would discourage the direct manipulation of the response cache unless you are trying to free up memory. As I mentioned over in #176, React Request is not meant to be a replacement of a normalized store like InMemoryCache, which impacted some of the design decisions like the limited ability to modify the cache.

[jamesplease]

I'm going to close this and consider the functionality here as something to be implemented alongside #176

To summarize my thoughts on these features: I'm open to it but won't have the time to implement it myself.