nix-env: Add regular expression support in selectors

So you can now do things like:

  $ nix-env -qa '.*zip.*'
  $ nix-env -qa '.*(firefox|chromium).*'

src/libexpr/names.cc

@@ -1,5 +1,6 @@
 #include "names.hh"
 #include "util.hh"
+#include "regex.hh"
 
 
 namespace nix {
@@ -32,7 +33,10 @@ DrvName::DrvName(const string & s) : hits(0)
 
 bool DrvName::matches(DrvName & n)
 {
-    if (name != "*" && name != n.name) return false;
+    if (name != "*") {
+        Regex regex(name);
+        if (!regex.matches(n.name)) return false;
+    }
     if (version != "" && version != n.version) return false;
     return true;
 }