Add CSS selector matcher

Given the following `input`:

>

Matching on title or value will fail.

```
$x('//input[@title="Email" or @value="Email"]')
> []
```

Place holder matching works but it's not unique.

```
$x('//input[@title="Email" or @value="Email" or @placeholder="Email"]')
> [​,
​]
```

I think we need to allow matching on arbitrary XPath selectors since there's no way to control the content of the webview.

The issue names no files, tests, or entry points. Start by locating the existing selector-matching code in the Objective-C iOS automation project and review issue #211's discussion; done requires resolving whether CSS or arbitrary XPath selectors are supported and distinguishing the two example inputs.

Written by the indexing model from the issue text.