Transducer init and result optional

[kevinbeaty]

Consider making @@transducer/init and @@transducer/result optional when passed an external transformer in into, transduce, etc. tResult could be identity by default, tInit could return new this.constructor().

[kevinbeaty]

tInit could return new this.constructor().

Although this would only work if tInit was defined on the prototype, in which case arguably return this would be a better default per discussion in #49 (ie. should [tInit]() return an empty object or equivalent object)