Rule: Warn on leading/trailing decimal points

We should warn when a numeric literal with a leading or trailing decimal point is encountered. For example:

```javascript
var x = .5,
    y = 2.;
```

See http://jslinterrors.com/a-leading-decimal-point-can-be-confused-with-a-dot-a/

This pull request adds the `raw` option to Esprima, as it's necessary to inspect the actual string as it appears in the source, rather than the numeric representation of it available in the `value` property of the node.