Narrow typing of loadCKEditorCloud result

[Mati365]

Suggested merge commit message (convention)

Feature: CKEditorCloudResult now returns more narrowly typed attributes for CKBox and CKEditorPremiumFeatures, taking into account CKEditorCloudConfig. Closes #24

Additional info

const { CKEditorPremiumFeatures } = await loadCKEditorCloud( {
  version: '43.0.0',
  premium: true,
  ckbox: {
    version: '2.5.1'
  }
} );

Before:

CKEditorPremiumFeatures is nullable type.

After:

CKEditorPremiumFeatures is not nullable type because premium: true is passed.

[gorzelinski]

Types seem to work fine, so I only added suggestions to the comments.

[Mati365]

@gorzelinski done