Include tiny-emitter in events API migration strategy

[yuripave]

In the migration guide, it suggest using mitt to migrate existing event hubs, but its interface is incompatible with Vue 2 where:

1. mitt's emit doesn't support variadic arguments that $emits supports
2. mitt doesn't support $once

I suggest to include tiny-emitter which its interface is fully compatible with Vue 2 $on, $off and $once, and its emit method do support variadic arguments.