Support module loaders

[mpschaeuble]

If Leaflet.Path.Drag is used with a module system, a packager like webpack cannot resolve the dependencies if this plugin has no support for it. As a result, the plugin cannot be used as the script loading order cannot be determined.

Please add support for AMD and Node/CommonJS module systems.

[w8r]

I will fix it.

[davidbeers]

Any progress on this, @w8r? Leaflet.Path.Drag is a plugin I would really like to be able to use, but I am prevented by not being able to use it as a module.