Config access for pattern engines

[geoffp]

This is the simplest and least disruptive way I can think of to make engines configurable. If supported by each engine, call a function that injects the global pattern lab config into the closure scope of the engine so that engines can make configurable decisions.

@bramsmulders and I are thinking this is necessary to be able to configure the React engine -- the first use case being the ability to shut off client-side code generation if you don't need interactive components and / or are just using React components to make a static style guide. There should be some build speed benefit to doing that.

This might also benefit the Handlebars engine, I seem to remember that it has some things you can configure.

See pattern-lab/patternengine-node-mustache#8 for the Mustache implementation.

[coveralls]

Coverage decreased (-0.03%) to 63.761% when pulling 15f89eb on config-access-for-engines into 15c456c on dev-3.0.

[bmuenzenmeyer]

I love this, the reference PR pattern-lab/patternengine-node-mustache#8 (which I might close rather than accept) and greatly appreciate the work.

[bramsmulders]

Whoop!