Establish consistent naming convention for all hooks in Vue

In lifecycle methods and the transition system, before/after prefixes (with camelCase) seem preferred instead of pre/post. For consistency and also because “before” and “after” are probably more accessible to Vue’s non-native English speakers, perhaps postupdate should be renamed to afterUpdate.

[yyx990803]

I’d like to avoid big naming changes for the sake of API continuity, so the goal is

1. consistency;
2. as few changes as possible.

What I see right now that feels inconsistent are:

1. postupdate hook in custom directives
2. onEnter and onLeave for transitions

I think if we fix these we could settle on the following:

1. present tense verb (bind, update, enter): indicates the user is implementing this hook to perform the specified action.
2. past tense verb (created, mounted): indicates the action was performed by the framework and the hook is exposed for its timing.
3. camelCase prefixed verb (beforeMount, afterEnter, enterCancelled): also indicates the action was/will be performed by the framework and the hook is exposed for timing purposes.

Based on these rules, the changes would be:

1. postupdate -> componentUpdated

2. onEnter -> enter, onLeave -> leave

   The reason I changed them to onEnter and onLeave in the first place was due to appear, which is a boolean, not a hook. However I think we can just determine the need for appearing transition simply by testing the presence of appear, afterApear and beforeAppear, and appear can either be true or an actual hook.

[yyx990803]

Implemented in next branch.