Swig support for custom trans/blocktrans tags

We're using [swig](http://paularmstrong.github.io/swig) together with currently not open sourced tags called "trans" and "blocktrans". They are used as following:
```
// trans
{% trans "This should be translated %}

// blocktrans
{% blocktrans foo='bar' %}
<a href="/">checkout %(foo)s profile</a>
{% endblocktrans %}
```

In order for jsxgettext to detect this, I had to add a custom swig parser as you see.
I know this is a pretty limited pull request as one who wants to use it needs to have the same 2 tags as we use (we plan to opensource them) and also a way to set tags to the swig instance.

Do you think it is possible to merge this PR in or do you think it doesn't fit because it's not generic enough?

- Michael