support nixos command-not-found, closes #912 (#922)

thefuck/rules/nixos_cmd_not_found.py

@@ -0,0 +1,15 @@
+import re
+from thefuck.specific.nix import nix_available
+from thefuck.shells import shell
+
+regex = re.compile(r'nix-env -iA ([^\s]*)')
+enabled_by_default = nix_available
+
+
+def match(command):
+    return regex.findall(command.output)
+
+
+def get_new_command(command):
+    name = regex.findall(command.output)[0]
+    return shell.and_('nix-env -iA {}'.format(name), command.script)

thefuck/specific/nix.py

@@ -0,0 +1,3 @@
+from thefuck.utils import which
+
+nix_available = bool(which('nix'))