Added a '--with-emacs' option to install emacs frontend

Added a '--with-emacs' option which requires emacs to be installed
through Homebrew. It then proceed to install the notmuch emacs frontend.

Library/Formula/notmuch.rb

@@ -5,13 +5,27 @@ class Notmuch < Formula
   url 'http://notmuchmail.org/releases/notmuch-0.13.2.tar.gz'
   sha1 '368b2451a64b1e3c574e688100700fc941ff2ea1'
 
+  def options
+    [
+     ['--with-emacs', "Compile with (Homebrew) Emacs support"]
+    ]
+  end
+
   depends_on 'xapian'
   depends_on 'talloc'
   depends_on 'gmime'
 
+  if ARGV.include? '--with-emacs'
+    depends_on 'emacs'
+  end
+
   def install
-    # requires a newer emacs than OS X provides, so disable the bindings
-    system "./configure", "--prefix=#{prefix}", "--without-emacs"
+    if ARGV.include? '--with-emacs'
+      system "./configure", "--prefix=#{prefix}"
+    else
+      # requires a newer emacs than OS X provides, so disable the bindings
+      system "./configure", "--prefix=#{prefix}", "--without-emacs"
+    end
     system "make install"
     system "install_name_tool", "-change", "libnotmuch.2.dylib",
                                 "#{lib}/libnotmuch.2.dylib", "#{bin}/notmuch"