Possibility to clear cache from custom memoize-function

[schneckentempo]

I would need the possibility to clear the cache at a certain point of the application. In this particular situation lodash memoize is used.
Would be nice if you could expose the memoizedResultFunc

[sgrishchenko]

This also helps write unit-tests.

[markerikson]

Resolved by #519 , and out in https://github.com/reduxjs/reselect/releases/tag/v4.1.0-beta.0 !