formula: add new methods

* `Formula#linked?` returns true if formula linked
* `Formula#optlinked?` returns true if formula linked to opt
  formula installed to the Cellar
* `Formula#prefix_linked?` returns true if linked keg points to the prefix
  passed as an argument
* `Formula#linked_version` to get linked version of the formula

Library/Homebrew/formula.rb

@@ -553,6 +553,28 @@ class Formula
     Pathname.new("#{HOMEBREW_CELLAR}/#{name}/#{v}")
   end
 
+  # Is the formula linked?
+  def linked?
+    linked_keg.symlink?
+  end
+
+  # Is the formula linked to opt?
+  def optlinked?
+    opt_prefix.symlink?
+  end
+
+  # Is formula's linked keg points to the prefix.
+  def prefix_linked?(v = pkg_version)
+    return false unless linked?
+    linked_keg.resolved_path == prefix(v)
+  end
+
+  # {PkgVersion} of the linked keg for the formula.
+  def linked_version
+    return unless linked?
+    Keg.for(linked_keg).version
+  end
+
   # The parent of the prefix; the named directory in the cellar containing all
   # installed versions of this software
   # @private