Fix topojson object path

[Conengmo]

Closes #828

Fix an issue with Topojson object paths. With the current code each item in the path should be a valid Javascript identifier, but users may not know this.

I looked into JS identifier validation, but I don't think that's the way to go:

• Topojson doesn't demand the objects have valid JS identifiers as names.
• There are many ways in which a JS identifier can be invalid, so it's hard to test.
• Users may be unable to change their Topojson data easily.

Instead I opted to convert the path to an array notation: obj1.obj2 -> ["obj1"]["obj2"]. After we escape double quotes any string should work. With this, object paths that include hyphens work.

Alternatively, only look for hyphens and raise an error if we see one. I'd rather fix the complete problem instead of just one specific case. But if this change is too complex, that might be a small step to take instead.

[Conengmo]

@ocefpaf or @martinfleis, would one of you mind reviewing this change?

[martinfleis]

This looks good.

Alternatively, only look for hyphens and raise an error if we see one. I'd rather fix the complete problem instead of just one specific case. But if this change is too complex, that might be a small step to take instead.

It doesn't seem to be an overly complex solution. I am fine with it and agree that it is better to fix the whole issue where it starts rather than a specific subset of it.

[Conengmo]

Thank you! Appreciate the review!