Read browserify options from yaml configuration file

As commented on https://github.com/defunctzombie/zuul/issues/41#issuecomment-64281720, this pull adds flexibility to zuul when configuring browserify with the default builder.

I can add documentation updates if given the go-ahead. For usage see: [zuulnice][1]

[1]: https://github.com/bevacqua/zuulnice
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