Solve the issue of geojson attribute visualization

[natsuapo]

The issue is mentioned here: #146

I modify the extract part. I think it can pass most of the test. But I am not sure whether there are still potential fails.

[natsuapo]

I found that this version of topojson cannot be read by some topojson libraries like the JavaScript one. I thought it is something wrong in my code, but I find that the original one cannot do that too. I am checking this part.

I found the issue is not related to this library, so I keep this merge request.

[mattijn]

Thanks, this is great. Much appreciated

[mattijn]

Hm, a bit too quick. There were indeed still some tests failing. I cannot decipher upon first glance if these are false flags, but before this PR all tests were still passing.

[mattijn]

I've reverted my merging. I'm not sure exactly if you still can add commits to this PR or if you have to open a new PR, since this is marked as merged.

[natsuapo]

I am sorry, I thought the test failing was just the path problem. I will test more.