mkogg.py: don't require explicit methods for conversion via wav.

mkogg.py: don't require explicit methods for conversion via wav.

Only require explit conversion methords for * -> wav and wav -> *.
With this and metadata getters/setters, conversion will work fine, and
we the number of methods will scale better with an increasing number
of file types.

You should still write a direct conversion method if you can avoid the
wav intermediate.