Support ECMAScript versions as `targets`?

> Issue originally reported by @rauschma in babel/babel-preset-env#381

I’m not sure how much that would be used in practice, but for some experiments and tests, it would be nice if one could specify an ES version to target.

Examples:

```
"targets": { "ecmascript": "2015" }
"targets": { "ecmascript": "5" }
```

Start by reviewing Babel's existing target handling and the referenced babel-preset-env#381 discussion. Compare the proposed ECMAScript 2015 and ES5 target forms with current behavior, and define completion as accepting those target values and applying the expected transformations in the relevant tests.

Written by the indexing model from the issue text.