Test tag value against tag value.

A new syntax is added to allow you to compare the value of
two different tags.

Example:

  name = $name:en { ... }

matches if the english name is the same as the plain name.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2814 25d90789-57f7-4ee0-8453-03a3dfeeeb22