fix(scoped-css): whitelist data-v-* attributes

[simllll]

purgecss 2.x removes unused classes when attributes do not match. this is the case when scoped css is used, therefore whitelisting of data-v-* is required.

this is only needed for "webpack" mode, as with the purgecss mode the classes are not scoped yet.

https://github.com/FullHuman/purgecss/issues/277#issuecomment-581475910

Beware, depends on https://github.com/FullHuman/purgecss/pull/279 which is not merged.

[simllll]

Depending pr has landed and new release is already out!

[manniL]

Thank you ☺️