emacsWithPackages: support installing larger packages

I was getting the following error building tide from Melpa:

nix-build -E '(import <nixpkgs> {}).emacs25WithPackages (p: [p.melpaPackages.tide])'

File tide-20170509.1134.tar is large (10.2M), really open? (y or n) Error reading from stdin
builder for ‘/nix/store/gs9ik7yf8iilsikkfing74i70m0diax3-emacs-tide-20170509.1134.drv’ failed with exit code 255
cannot build derivation ‘/nix/store/m3p080aani4rw82llp8nqk93cw2nvirk-emacs-with-packages-25.2.drv’: 1 dependencies couldn't be built

Solution was to disable the large file warning threshold when
installing packages.

pkgs/build-support/emacs/elpa2nix.el

···
         (insert-file-contents file))
       (when is-tar (tar-mode))
       (elpa2nix-install-from-buffer))))
+
+;; Allow installing package tarfiles larger than 10MB
+(setq large-file-warning-threshold nil)