#10 Add flags to disable certain dependencies

In my opinion other packages should import finitary and provide instances, instead of having finitary incur dependencies.

I've taken the liberty to add some cabal flags that allow users to turn off these dependencies. I've checked that the library builds without warnings with all 4 combinations of the two flags I added.

Up to you whether you want these flags turned on or off by default. I've left them on, so that passing no flags recovers the current behaviour.

Thanks for the PR! Unfortunately, I'm no longer working on finitary. If you'd like the package, let me know.