Add ability to set voice globally?

[joshterrill]

Is this something that is already being worked on by chance? If not I can make a pull request.

[tlhunter]

This is a great idea.

I do have some ideas for a new interface at https://github.com/Marak/say.js/issues/63.

Being able to set each of those options as defaults at time of instantiation and only provide text when calling .say or .export does make a lot of sense.

I think we should try to tackle the default options at the same time as we modify the interface, as they'd be fairly large changes, so maybe hold off for now.