Motivation for this change

Sometimes it's needed to override parts of vim_configurable, for
instance when using ENSIME (http://ensime.github.io/), in this case you
need a Python interpreter and the modules sexpdata and
websocket_client.

However overriding vim_configurable is quite hard as we run
vimUtils.makeCustomizable over the default comming from
configurable.nix. Therefore it's necessary to copy the code from
all-packages.nix and alter the parts you need: https://nixos.org/nix-dev/2017-April/023364.html

In order to simplify overriding vim_configurable I added an
override and an overrideAttrs function to vimutils.makeCustomizable
to ensure that the customization capabilities won't be lost after
altering the derivation.

Now it's possible to write expressions like this without evaluation
failures:

with pkgs;

let
  vimPy3 = vim_configurable.override {
    python = python3;
  };
in vimPy3.customize { ... }

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.