Allow use array syntax in keep-alive include/exclude

[jkzing]

What problem does this feature solve?

The comma-delimited string syntax is not so intuitional, and actually it is splitting the string into an array. So why not support array syntax directly?

What does the proposed API look like?

<keep-alive :include="['a', 'b', 'c']">
</keep-alive>

[Kingwl]

merged in #5956