Rewrite source

This change set replaces `.updateExports()` with a simpler method called `.update()`.  This new method takes a new vertex/fragment shader runs them.

If there is a down vote on this, then I will merge this change set into `gl-shader`.

Longer term, I think that maybe glslify should move to `sourceOnly` as the default and only option, then the particular choice of shader wrapper wouldn't matter as much.