Support private language tags

Please provide the following details if possible or relevant:

- [] the version of Ace impacted by the issue (as returned by the `ace -v` command).
1.0.0

Some of our ebooks feature italics for "alien" languages. When used in that way, we include a private language subtag in the source. For example, in [The Moon Pool](https://standardebooks.org/ebooks/abraham-merritt/the-moon-pool):

````

“Hold up ten times twenty of our corials with but two fingers⁠—and these corials of ours⁠—”

In the IANA spec, `x-` denotes that the language tag is in "private" use; that is, it is known that the language tag does not correspond to valid existing language tag. However, Ace outputs this error when it encounters these private-use subtags:

> Value of lang attribute not included in the list of valid languages

I think Ace should probably ignore private language subtags for accessibility-checking purposes.