phonetisaurus-train support for alternative LM toolkits

[AdolfVonKleist]

The purpose of this issue is to extend the phonetisaurus-train convenience script to support a wider range of well-known LM training toolkits, in addition to the backoff inclusion of a simple native alternative.

Extending to each of these toolkits should only require a series of appropriate shell commands corresponding to these, which can be interpolated appropriately into phonetisaurus-train:

• https://github.com/AdolfVonKleist/Phonetisaurus/blob/master/src/bin/phonetisaurus_train#L135

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• mitlm - ✓
• rnnlm - ✓ (native)
• pocolm - x
• native - x
• srilm - x
• kenlm - x
• irstlm - x
• OpenGrm-NGramLibrary - x (legacy implementation exists)