Breaking: support ESLint 5 and drop the supports of old stuffs

[mysticatea]

Fixes #398, fixes #423, fixes #521, closes #504.

This PR does:

• Upgrades vue-eslint-parser to 3.x to support ESLint 5 and fix several bugs. See v3.0.0 to check changes. Notable changes are:
  • The AST of slot-scope and <template scope> attributes was changed. Those attributes are now parsed to VSlotScopeExpression node.
  • The parserOptions.ecmaFeatures.experimentalObjectRestSpread option was removed as a result of upgrading espree to the latest.
  • The v-on directive with no argument and an object expression (e.g., v-on="{foo: bar}") is now parsed to an ObjectExpression node correctly.
  • HTML void element list was updated (vuejs/vue-eslint-parser@7de2b86). Some tags of non-standard void elements now requires that end tags.
  • The parserOptions.parser option can be false. If it's false, the parser skips parsing <script> elements completely.
• Drops the support of Node.js 4.x (it was EOL already).
• Drops the support of ESLint 3 and 4.
• Fixes html-indent rule to support new VSlotScopeExpression node.
• Removes all parserOptions.ecmaFeatures.experimentalObjectRestSpread options in our tests.

Plus, we can move this plugin to @vue/eslint-plugin now. @vue shorthand has been supported since ESLint 5.0.0.

[michalsnik]

Awesome! We should however also mention that this README is about the upcoming version 5, and that for the last stable version people should go here: https://github.com/vuejs/eslint-plugin-vue/tree/4.x

[mysticatea]

I updated README.md with copying from dfcd26e.