Add support for ph attribute verification in phoneme tag

[gsdriver]

Splitting this off from #12

From my (limited) experience, the implementation is very picky and hard to debug, since every locale only supports a subset of the IPA phones, and the tag is completely ignored as soon as there's just one unsupported character in the ph attribute. The validation of ph would be a very useful feature, but also very complex. Maybe this should be a separate issue, or even be outsourced to a separate library?

see this archived page) for more details

[lenaschimmel]

Thanks for keeping this up-to-date!

Just a clarification: the "this archived page" link was only relevant to the feature being in beta / being out of beta. This context got lost during issue splitting. The current version of the API documentation is here.