Add top-level from_url function

[kylebarron]

Closes #90

Todo:

• Note: will not read from the environment. Now that we're passing to e.g. S3Store::from_url it should actually read from the environment! (thanks to Always check env when constructing stores, remove from_env #189) (note: we need to test this!!)
• Add test for reading from the env.
• Note: Cannot be used for sign
• Add to docs

We may want to vendor parse_url_opts (which is pretty small), so that we can return the Python classes directly, rather than returning a PrefixStore. At least if we switch to storing a prefix store. #184

[kylebarron]

This PR is waiting on #185, which will enable the prefix handling to work out of the box

[kylebarron]

@maxrjones what do you think about naming this

obstore.store.from_url

?

Then it's also less surprising that it delegates to from_url of each individual store.

[maxrjones]

@maxrjones what do you think about naming this

obstore.store.from_url

?

Then it's also less surprising that it delegates to from_url of each individual store.

I like it!