Add experimental-features setting

Experimental features are now opt-in. There is currently one
experimental feature: "nix-command" (which enables the "nix"
command. This will allow us to merge experimental features more
quickly, without committing to supporting them indefinitely.

Typical usage:

$ nix build --experimental-features 'nix-command flakes' nixpkgs#hello

(cherry picked from commit 8e478c2341,
without the "flakes" feature)

src/libstore/globals.hh

@@ -353,6 +353,11 @@ public:
 
     Setting<Paths> pluginFiles{this, {}, "plugin-files",
         "Plugins to dynamically load at nix initialization time."};
+
+    Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
+        "Experimental Nix features to enable."};
+
+    void requireExperimentalFeature(const std::string & name);
 };