Change `withShownDefault` and `valueWithShown` to accept a function

### [Change withShownDefault to accept a function](https://github.com/NorfairKing/opt-env-conf/pull/17/commits/1f3567bf35a4989186a3bb35669afebabe4fb927) [1f3567b](https://github.com/NorfairKing/opt-env-conf/pull/17/commits/1f3567bf35a4989186a3bb35669afebabe4fb927) It's common to use this function on a type with a custom "show-like" function. Under such usage, the ergonomics are not ideal: ```hs withShownDefault defaultFoo (showFoo defaultFoo) ``` This function is more ergonomic: ```hs withShownDefault showFoo defaultFoo ``` As an example, the simplified `withDefault = withShownDefault show` reads very clearly. The previous behavior can also be recovered easily with `const`. ### [Update valueWithShown to accept a function](https://github.com/NorfairKing/opt-env-conf/pull/17/commits/5bbbebd650b1c77f1b8f15784c9501463f3d5df0) [5bbbebd](https://github.com/NorfairKing/opt-env-conf/pull/17/commits/5bbbebd650b1c77f1b8f15784c9501463f3d5df0) The arguments for this are the same as the previous change to `withShownDefault`. ### [Version bump and CHANGELOG](https://github.com/NorfairKing/opt-env-conf/pull/17/commits/4f135bf3e639c03debc02c01976ca1389c5372d8)