Panels integration - Metatags add input format possibility

The fields are handled as plaintext and will stay that way, it'll be much more hassle dealing with unwanted formatting being added (accidentally or otherwise) to meta tag values.

There may be another way of resolving encoding problems, but it won't be through using input formats. Feel free to open another issue to deal with the encoding problems if it is something you can help debug as I've personally not come across it before.