Selector.oneOf

@avh4 [proposed](https://github.com/eeue56/elm-html-test/issues/32#issuecomment-306840487):

> some way to write a query that will find the button when given `a`, and the input when given `b`:

```elm
a = div [] [ button [] [ text "Click me" ] ]
b = div [] [ input [ type_ "button", value "Click me" ] ]
```

The use case would be writing a reusable function to detect equivalent UI elements—e.g. a `fancyButton` function that queries for buttons with either the class `smallFancyButton` or `largeFancyButton`.

He proposed two potential APIs. One was:

```elm
Query.find
[ Selector.oneOf
[ [ tag "button", text "Click me" ]
, [ tag "input", attribute "type" "button", attribute "value" "Click me" ]
]
]
```

The other potential API he proposed was:

```elm
case Query.toResult <| Query.find [ tag "button", text "Click me" ] of
Err _ ->
Query.find [ tag "input", attribute "type" "button", attribute "value" "Click me" ]
Ok single ->
single
```

The `oneOf` API seems like a good choice to me.

Start by reviewing the proposed Selector.oneOf API and the existing Query.find and Query.toResult approaches shown in the issue. Compare the two alternatives and determine which API fits the project; done means reusable queries can match either the button or input example and equivalent UI elements can be detected.

Written by the indexing model from the issue text.