Expose Request "config" and "transformers" fields in transform_wasm

This would help us with integrating into other projects, such as [amp-toolbox](https://github.com/ampproject/amp-toolbox).

Proposed valid invocations:

```js
transform('https://example.com/', html) // uses DEFAULT config

transform('https://example.com/', {
config: 'VALIDATION'
})

transform('https://example.com/', html, {
config: 'CUSTOM',
transformers: [
'ampboilerplate',
'urlrewrite'
]
})

Start at the transform_wasm entry point and review how transform currently accepts its arguments. Compare the supported calls with the proposed config and transformers forms; done means those fields can be supplied for the documented invocations and the existing default behavior remains available.

Written by the indexing model from the issue text.