mpv: fix issue #1725 and add tests (#1726)

Closes issue #1725.

This allows mpv module to be customized with support for more advanced
features than the `programs.mpv.scripts` current support. For example,
with this change now this is possible:

```nix
{
  programs.mpv.package = (pkgs.wrapMpv (pkgs.mpv-unwrapped.override {
    vapoursynthSupport = true;
  }) {
    extraMakeWrapperArgs = [
      "--prefix" "LD_LIBRARY_PATH" ":" "${pkgs.vapoursynth-mvtools}/lib/vapoursynth"
    ];
  });
}

```

Since `programs.mpv.package` doesn't necessary reflect the final
derivation anymore (see #1524), we introduce `programs.mpv.finalPackage`
that has the resulting derivation.

This includes 2 tests:
- One to check if everything is alright with mpv
- Other to validate our assertion that package and scripts can't be
  passed both at the same time

* docs: document recent mpv module changes

* mpv: add thiagokokada as maintainer

modules/lib/maintainers.nix

@@ -47,4 +47,10 @@
       fingerprint = "F0E0 0311 126A CD72 4392  25E6 68BF 2EAE 6D91 CAFF";
     }];
   };
+  thiagokokada = {
+    email = "thiagokokada@gmail.com";
+    name = "Thiago Kenji Okada";
+    github = "thiagokokada";
+    githubId = 844343;
+  };
 }