Add exclude options for leaflet dependencies

[robbiespeed]

Nice to have options, so that users of the addon can opt out of including the lealfet dependencies in favor of versions they choose to include themselves. For example the leaflet css contains some visual tweaks that might make it annoying to have to override. In order to use just add the desired options in the ember-cli-build.js file:

var app = new EmberApp(defaults, {
  'ember-leaflet': {
    excludeCSS: true,
    excludeImages: true,
    excludeJS: true,
  },
});

[miguelcobain]

@robbiespeed can you please add an entry in the changelog? This looks nice.

[robbiespeed]

@miguelcobain Thanks just added the entry

[miguelcobain]

Thank you, sir!