.dir-locals.el: init

This adds some initial values for .dir-locals.el. Mainly this is
useful for using bug-reference-mode.

So if you have bug-reference-mode enabled -

> M-x bug-reference-mode

You will see as clickable text like this:

  Fixes NixOS#15

  (NixOS#12)

  Closed NixOS#1252

  issue #1

.dir-locals.el

@@ -0,0 +1,8 @@
+;;; Directory Local Variables
+;;; For more information see (info "(emacs) Directory Variables")
+
+((nil
+  (bug-reference-bug-regexp . "\\(\\(?:[Ii]ssue \\|[Ff]ixe[ds] \\|[Rr]esolve[ds]? \\|[Cc]lose[ds]? \\|[Pp]\\(?:ull [Rr]equest\\|[Rr]\\) \\|(\\)#\\([0-9]+\\))?\\)")
+  (bug-reference-url-format . "https://github.com/NixOS/nixpkgs/issues/%s"))
+ (nix-mode
+  (tab-width . 2)))