regex syntax

I've been parsing some geojson files with this module. Annoyingly, some geojson files use "NAME" in the properties field, others use "Name", and still others use "name". This patch adds support for regex key names in both parse string syntax:

``` js
'features.*.{name: properties./^name$/i}'
```

and in object syntax:

``` js
['features', true, { name: ['properties', /^name$/i] } ]
```

RegExp keys are already handled by JSONStream, so this patch is fairly minimal.