Simplified API prototype

Hi,

I have been writing some golden tests using tasty-golden, but I am not really happy with the API. The problem I faced was that it was either to low-level (Golden/Advanced.hs) or to high-level (Golden.hs).

I would also like some sort of interactive mode, where it asks me if I want to accept the test result as new golden value if they differ.

I did try do that, but this required some larger changes. The new [API](https://github.com/phile314/tasty-golden/blob/5caaf4c6834b645800d2935a2bd366705b30cdea/Test/Tasty/Golden.hs) basically assumes all input is valid (UTF8) Text, and it also provides some help in capturing stdout,stderr and exitcode all together. I have an example at 
[https://github.com/phile314/agda/blob/compiler-tests/test/exec/Main.hs#L73-L119](https://github.com/phile314/agda/blob/compiler-tests/test/exec/Main.hs#L73-L119).

 This is a work in progress right now, but would you be interested in merging this at some point?

Cheers,
Philipp