Handle when `yuidocOptions` is missing and allow falsey values.
Handle when `yuidocOptions` is missing and allow falsey values.

When `yuidocOptions` is undefined an error is thrown (`Cannot read
property 'linkNatives' of undefined`).

Also, the prior version would not allow falsey values to override the
defaults.

mixonic/ember-cli-yuidoc,VladimirTyrin/ember-cli-yuidoc,VladimirTyrin/ember-cli-yuidoc,greyhwndz/ember-cli-yuidoc,cibernox/ember-cli-yuidoc,mixonic/ember-cli-yuidoc,greyhwndz/ember-cli-yuidoc,cibernox/ember-cli-yuidoc