Add experimental-features setting

Experimental features are now opt-in. There are currently two
experimental features: "nix-command" (which enables the "nix"
command), and "flakes" (which enables support for flakes). 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

src/libstore/globals.hh

@@ -356,6 +356,11 @@ public:
 
     Setting<std::string> githubAccessToken{this, "", "github-acces-token",
         "GitHub access token to get access to GitHub data through the GitHub API for github:<..> flakes."};
+
+    Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
+        "Experimental Nix features to enable."};
+
+    void requireExperimentalFeature(const std::string & name);
 };