Add the ability to touch keys on read about cachex

[whitfin]

I'm not sure about having it as a cache-based option because there are places I don't think it should be enforced (for example ttl/3). It might make sense to make it an option instead.

I imagine it would only make sense for it to be used with either get/3 or exists?/3, but I think I'd write it into read/2 in the workers and only pass the options through as they're needed.

[whitfin]

As of this point I don't think this is particularly valuable beyond sugar. You can accomplish the same using refresh/3 and it's hard to predict what the user would expect if it were to work automatically - it might then become a request to add automatic refresh onto things like get_and_update/4 which would cause more pain than worth to embed.

As I'm not including this at this point, here are some notes on how it can be achieved (using get/3 as an example - but same applies to anything else). Naturally you might want to do some better error handling and what not:

Cachex.transaction(:my_cache, fn(worker) ->
  case Cachex.get(worker, "my_key") do
    { :ok, _val } = result ->
      Cachex.refresh(worker, "my_key") && result
    result ->
      result
  end
end)

Remember that if you're working on a local cache it might be okay to use execute/3 rather than transaction/3 for improved performance.