The `StyleObserver` constructor doesn't accept single values as the values of the `properties` and `targets` options

[DmitrySharabin]

Found by @evayde in #135.

Although the documentation mentions that targets and properties can be either a single value or an iterable,

this is true only for the observe() and unobserve() methods. I wonder if it's a bug in the docs or the constructor? 🤔

[evayde]

What would be the issue with supporting both? Other than the weird naming (assigning a single value to a property that has a plural in its name)

[LeaVerou]

This seems like a bug in the constructor to me. It should support both if observe() supports both.