Update: improve warnings about key on template elements

[mysticatea]

Fixes #43.

This PR does 2 things.

1. Fixes no-invalid-v-for and require-v-for-key rules to check whether child elements of <template> elements have v-bind:key attribute. [semver-minor]
2. Add new no-template-key rule to disallow key attribute on <template> elements. I think that no-template-key rule should be recommended, however I set recommended: false for now because semver policy of ESLint needs a major version to update the recommended config. [semver-minor]

[kazupon]

I think GitHub emoji code (e.g. :tada:) is maintainaceble(read/write) than emoji binary code. 👀

[michalsnik]

Agree with @kazupon, I did so while importing elint-plugin-vue-trial too

[mysticatea]

In my custom, I don't use the notation since it doesn't work outside of GitHub (i.g. document generators, editor previews, etc...). Should I use it?

[mysticatea]

I solved merge conflicts.

[michalsnik]

Looks good @mysticatea However I don't quite get why the key on template is considered harmful, could you please explain this a little bit? Reason behind this in rule's documentation would also be helpful for others I think.

[armano2]

thats why: #43
<template> cannot be keyed. Place the key on real elements instead.