refactor(JSX::Transformer) use Transformer class; allow asset_path config

[rmosolgo]

Simplifying the JSX module and providing the feature from #239

• Use an instance of JSX::Transformer for transforming JSX
• JSX.transform_options are initialization options for Transformer
• feature: allow custom JSX files by passing asset_path: config (or even overriding Transformer#jsx_transform_code 😬 )

[rmosolgo]

I added that clear_sprockets_cache helper to the tests, one of these days I'll go through and weed out all the different ways that the cache is cleared and replace them with that helper