Allow onFinished to turn off default endOnError behavior.

[vqvu]

Fixes #532.

Allows implementations of onFinished (when constructing a Highland stream from a Readable) to turn off the default behavior via continueOnError.

 _(req, function (req, callback) {
     req.on('error', callback);
     return {
         onDestroy: function () {
             req.removeListener('error', callback);
         },
         continueOnError: true
     };
 }).pipe(writable);

[svozza]

Is _rs just a placehlder value here?

[vqvu]

Yes. The linter doesn't like it when I shadow variables in functions. Both _rs and rs should reference the same Readable.

[svozza]

Cool. Makes sense.