tig: add '--with-docs'

Closes Homebrew/homebrew#22946.
Closes Homebrew/homebrew#23270.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/tig.rb

@@ -5,13 +5,20 @@ class Tig < Formula
   url 'http://jonas.nitro.dk/tig/releases/tig-1.2.1.tar.gz'
   sha1 '5755bae7342debf94ef33973e0eaff6207e623dc'
 
-  depends_on "asciidoc"
-  depends_on "xmlto"
+  option 'with-docs', 'Build man pages using asciidoc and xmlto'
+
+  if build.with? "docs"
+    depends_on "asciidoc"
+    depends_on "xmlto"
+  end
 
   def install
     system "./configure", "--prefix=#{prefix}", "--sysconfdir=#{etc}"
     system "make install"
-    system "make install-doc-man"
+
+    if build.with? "docs"
+      system "make install-doc-man"
+    end
 
     bash_completion.install 'contrib/tig-completion.bash'
   end