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

1 files changed, 2 insertions, 0 deletions

diff --git a/src/nix/main.cc b/src/nix/main.cc
index eedf8656e..8cdeed8f5 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -149,6 +149,8 @@ void mainWrapped(int argc, char * * argv)
 
     args.parseCmdline(argvToStrings(argc, argv));
 
+    settings.requireExperimentalFeature("nix-command");
+
     initPlugins();
 
     if (!args.command) args.showHelpAndExit();