Add support for runtime substitution (@eval)

For the time being,  to add a runtime substitution you have to define an at-rule like:  
```
@eval MY_EVAL "com.foo.Bar.foo()";
```

The double quotes (or single quotes) are mandatory otherwise the parser throws a ParsingException

After the discussion of this morning, I think the best will be to use a function : 
```

@def MY_CONSTANT eval('com.foo.Bar.foo()');
````