Allow unsuppressed comments to be compiled from nested imports

Fixes #444

Since imports are parsed as Block nodes and Block nodes previously
only compiled comments when properties were present, imported comments
were being lost. This fix adds comment compilation for Block nodes
without properties as well, but only if the Block does not have scope
(as is the case with an import).

In order to prevent comments from existing plugins and libraries
(like Nib) from being compiled, suppressed comments (without '/*!') are ignored.