Add a new tag to transclude version values

I've added a version directive. {{VERSION}}, {{V.V.V}}, {{V.V}} and {{V}} are replaced in a file by the version number of the given cardinality (major.minor.dot). {{VERSION}} is just a clearer way of denoting {{V.V.V}}.

eg. if your markdown text contained

```
You can download Riak version {{VERSION}} from
http://s3.amazonaws.com/downloads.basho.com/riak/{{V.V}}/{{V.V.V}}/riak-{{V.V.V}}.tar.gz
```

it wil be replaced by this in Riak version 1.2.1

```
You can download Riak version 1.2.1 from
http://s3.amazonaws.com/downloads.basho.com/riak/1.2/1.2.1/riak-1.2.1.tar.gz
```