This feature enables users to select a preset/flavor.
A flavor is just a preset of options, a shortcut so users don't have to set each option one by one.
Closes#164
Old extensions that register themselves in `showdown.extensions` can be loaded and validated using the new extension loading mechanism.
However, a warn is issued, alerting users and developers that the extension should be updated to use the new mechanism
BREAKING CHANGE: Deprecates `showdown.extensions` property. To migrate, you should use the new method `showdown.extension(<ext name>, <extension>)` to register the extension.
