Allow custom link text for urlize filter

My templates usually contain a lot of the following pattern:

    <a href="{{ object.url }}">{{ object.name }}</a>

This commit adds an optional `link_text` argument to the urlize filter:

    {{ object.url|urlize:object.name }}

The pull request contains the implementation and tests. If you want to include this in Django, I'll add a commit to this pull request that updates the documentation.