Issue #1768 - allow spacing for [ when following '::' operator.

This allows '[' to be preceded by whitespace and still get recognized when following the '::' operator.

In other words, 
```
Array:: ['isArray']
```
will now compile as expected (though I'm not sure I would have expected this!)

I didn't write a test, as I don't know how to do tests for coffeescript.  If you want me to write a test, I can try but I may need some help getting started.