Split multi-part STL

Sometimes one gets an STL file that has multiple parts in it, and one would like to extract the parts separately (e.g., recently, I wanted to extract a single link from someone's chainmail STL). One could use Meshmixer or something like that, but a simpler solution would be nicer.

I made a [simple javascript page](https://htmlpreview.github.io/?https://raw.githubusercontent.com/arpruss/stlutils/master/split.html) that does that. You just select a file, and then you can download the parts separately (or a combination of a subset of parts). The javascript runs all in your browser: nothing is uploaded to a server.

Running the splitter on a rendering of "STL Split" yields nine parts, one for each letter other than the "i", and two for the "i".

**Usage note**:
Currently, this will fail in a funny way if your object has an inner surface completely contained within an outer surface, namely a hole completely surrounded by material. The split will result in two meshes, one for the ou