snippets are no longer available via localStorage

We made snippets (among other settings) available from incognito. I think this change landed in Chrome stable around 46/47 or so.

As a result, they are no longer available in `localStorage`. Here's the new technique. These are run from a [devtools-on-devtools](http://stackoverflow.com/a/12291163/89484) window.
### getting snippets

``` js
InspectorFrontendHost.getPreferences(_ => console.log(JSON.parse(_.scriptSnippets)))
```
### setting snippets

``` js
InspectorFrontendHost.setPreference("scriptSnippets", JSON.stringify(yourSnippets))
```

FWIW, `InspectorFrontendHost.getPreferences` will return everything that has migrated to the incognito-available settings (most things), while a few items (like breakpoints) remain in the devtools' `localStorage`

Tagging a few other issues that use this technique:
- #6
- https://github.com/bgrins/devtools-snippets/issues/28
- https://github.com/bgrins/devtools-snippets/pull/35
- http://stackoverflow.com/questions/19025718/which-file-does-snippets-of-chrome-dev-tool-saved-at