Support for custom blocks

[wolfgangwalther]

eslint-plugin-vue-i18n uses vue-eslint-parser to load .vue files and do some linting of <i18n> custom blocks.

This breaks down with something like the following (see intlify/eslint-plugin-vue-i18n#107):

<i18n lang="yaml" locale="en">
unused: "<i"
</i18n>

This works with vue-loader, because the content of the custom block is not parsed as HTML.

If I understand correctly, the problem is that vue-eslint-parser tries to parse the whole document as HTML. The <i is invalid HTML and breaks this parsing. I guess the <script> tag is handled specially already. It would be good if vue-eslint-parser would handle custom blocks by returning the raw content instead of trying to parse them as HTML. The content of a top-level block should only be parsed as HTML if it's either a <template> block without lang attribute or if the lang attribute is set to lang="html".

This should also solve #63 maybe?