Standardise on how we include phonemisers and make them more widely available

[ftyers]

Not sure if this is the right place for it, but it would be cool to add phonemisers to Apertium in a standard way. E.g.

$ echo "sibʼalaj" | apertium -d . quc-phone
^sibʼalaj/siɓalaχ$

Espeak has a lot of phonemisers that I think are X-sampa. It would be cool to import those and IPA-ise them.