Read trees from URLs

[Wytamma]

We should handle reading trees from urls. Not sure what the API should be though...

[LeoFeatherstone]

I have added an example for how to do this with a fetch request in #63

[Wytamma]

This issue was about adding a feature to do this internally if a URL is passed. Currently PhyloJS will raise an error if something that looks like a URL is passed to read -> https://github.com/clockor2/phylojs/blob/main/src/io/readers/tree.ts#L19