Support ghc-8 (and update readme; ghc-7.10.3 seems to work fine)

[lspitzner]

[573]

Hello @lspitzner,

I just tried to build exference in a nix-shell using ghc-7.10.3 and it didn't work out for me.
The attempt in a gist is here.
Probably it is just a nix issue, not an exference one, let me know if we have to move this.

[lspitzner]

Looks like the versions of haskell-src-exts and lens in you nix-store are newer than what the exference.cabal currently permits. Upgrading probably is trivial in regards to lens, but for haskell-src-exts i just tested and things are not entirely trivial.

[lspitzner]

If it does not take too long, i might be able to adapt to new haskell-src-exts later today. Otherwise you may have to change your nix setup.

[lspitzner]

i started adapting to new haskell-src-exts, but i won't finish this today.

[vaibhavsagar]

My first stab at upgrading haskell-src-exts is here, is it along the right lines?