Custom tags

So, I'm just going to add a request for this, because I think it's important to consider.

Custom meta tags would be useful for several reasons, among them:

1. The metatag module maintainers cannot be expected to know the full details of every meta tagging scheme, and this would fill in the gaps.
2. Implementing ALL the tags, for all the tagging schemes, for each node type and vocabulary, would make the metatag administration pages VERY long and confusing (even with the great UI that now exists - good job Dave), and this would allow a set of core tags to be defined for each scheme to which users could add as needed.
3. We're programmers! We like to cover all possibilities!

The major problem for this, as I understand it, is that different schemes have to be in different formats, and these require different handlers. However, it seems that, for example, all opengraph tags are formatted the same and use the same handler. So, what I'm thinking is that each submodule (like metatag_opengraph) could define different types of tags (e.g. Opengraph tag) and then the main metatag module could allow people to implement custom tags of that type (for example, og:isbn, if anyone actually uses that). I could be wrong, and that's fine, but I just wanted a place for this question.