Support linting yaml i18n blocks in SFC

[wolfgangwalther]

Now that linting <i18n> blocks in SFC is possible (#80), it would be great if yaml would be supported as well.

Using yaml for i18n blocks is suggested in the vue-i18n docs at https://kazupon.github.io/vue-i18n/guide/sfc.html#yaml-loading, so I guess it would make sense to support it here as well.

Maybe it's possible to just load the <i18n> blocks through vue-loader as suggested in #80 (comment) instead of loading the json manually? That might take care of all that automatically.

[wolfgangwalther]

Just tested it - works for no-missing-keys and no-unused-keys! Great!

It didn't work for no-html-messages though. I put html code into a SFC block and it didn't throw.

Anyway, thanks a lot for adding this!!