Rule proposal: `v-if-else-key`

[michalsnik]

Style guide:
https://vuejs.org/v2/style-guide/#v-if-v-if-else-v-else-without-key-use-with-caution

Description:
This rule would enforce to add key property on elements with v-if and v-else directives if they're both of the same type.

[chrisvfritz]

This is very much related to our discussion here. The same requirements regarding key with v-for exist for v-if, so maybe these should even be in the same rule, perhaps called require-key-on-stateful-enumerated-elements?