Support variables in imports.

This branch add support for variable interpolat in imports.

It is based on #39 . Hope the parent will be merged so that diff will be much smaller.

There is still a problem with variables declaration as the value is somethies a list and sometimes not. See failing tests from test_variable.

It add a new token less_variable_interpolated to make the distiction betwen the case when a variable is defined and where it is interpolated.

I have pushed it just to know that I am working on this and would like to see it fixed.

Hope that with more unit tests we can unity the behaviour of variable parsing.

Thanks.
Adi