Implement Transformer.splice() about pysox

[terencecollins]

+1: the documentation for fade says only "see splice". but it seems that there's a command in soxmix (and hopefully, therefore, sox) to implement splicing with cross-fades: https://linux.die.net/man/1/soxmix.

[terencecollins]

One other thought: what if there was a build_arg command, which return the arguments to sox(), after which one could extend either returned arg list or self.effects with arbitrary arguments. Might go a long way to future proofing for any additional functionality added to sox.

[turian]

@rabitt would love this feature. I am looking for a good programmatic splice with crossfade and pysox seemed like a good option.