When loading plugins, adds a check of the package.json config variable "generator-core-version"

If this value is present, and also not compatible with the current version of generator-core (read from package.json), then Generator will not load the plugin. Instead, an alert will be shown in Photoshop.

For a plugin to implement this, its package.json should look something like:

```javascript
{
  "name": "generator-assets",
  "generator-core-version": "~1"
  // other stuff
}
```

Fixes #79