gopass: Install shell completion scripts

Shell completion needs to be generated by the `gopass` binary, and moved
into place for each of the supported shells.

pkgs/tools/security/gopass/default.nix

@@ -20,6 +20,16 @@ buildGoPackage rec {
     gnupg
   ]);
 
+  postInstall = ''
+    mkdir -p \
+      $bin/share/bash-completion/completions \
+      $bin/share/zsh/site-functions \
+      $bin/share/fish/vendor_completions.d
+    $bin/bin/gopass completion bash > $bin/share/bash-completion/completions/_gopass
+    $bin/bin/gopass completion zsh  > $bin/share/zsh/site-functions/_gopass
+    $bin/bin/gopass completion fish > $bin/share/fish/vendor_completions.d/gopass.fish
+  '';
+
   postFixup = ''
     wrapProgram $bin/bin/gopass \
       --prefix PATH : "${wrapperPath}"