Add optional `lang` information to headers?

Part of keymanapp/keyman.com#383 where we should add more HTML lang attributes to the appropriate pages.

Should we make a way for MarkdownHost to specify a language for a Markdown page?
Would the yaml be something like?

```yml
---
language: my
---
```

I think the PHP headers are handled separately for each site.

🔗 keymanapp/help.keyman.com#1668