Use `property` as `meta` key too

Currently, only `name` is considered as a valid key for <meta>,
but more and more website, trying to be mobile-friendly,
are using <meta> tags without `name`, but with a `property`
instead. This commit changes this behaviour.

This idea emerged from [this MR](https://github.com/AliasIO/Wappalyzer/pull/1321)