Added "sinc" effect, and a "custom" argument

[hadware]

I hope i didn't get too crazy on the inputs checking part on the sinc effect (tried to make it as clear as possible, since the original SoX doc is pretty confusing IMHO).

Also, I figured it'd be better to add a "custom" method to the effects chain, which would enable you to add your own effect string (pretty useful for effects that are not yet implemented). Since now all unimplemented effects can still be used, I think we sould remove all the "empty" methods in on the main object.

[carlthome]

Status?

[hadware]

I fixed most of what you requested. I still need to add the custom method to the unit tests like you requested

[carlthome]

Thanks! 😃