Convert.export doesn't export integer ids for arc labels when using symbol tables as openfst expects

[steveash]

When exporting openfst/AT&T fst text versions of the FSTs the ilabel and olabel symbols in the fst file should be the ids of those symbols from the symbol tables -- not the symbols (strings) themselves. for backwards compatability this openfst-compatability mode should be opt-in (like omit non-final symbols is).