Enable to configure any Koa.js middleware

[Yusaku Hatanaka]

Background

In the current rendertron implementation, when it starts, Koa.js middlewares below are configured.
https://github.com/GoogleChrome/rendertron/blob/main/src/rendertron.ts#L44-L48

But each developer can't add any Koa.js middleware and configure any options to existing Koa.js middleware.

For example in my use-case, I'd like to output JSON format log but currently, rendertron configures koa-logger.
It doesn't support JSON format, so I'd like to use bunyan-logger or cabinjs instead of koa-logger

Expected behavior

Developer can configure any Koa middlewares instead of existing middlewares

[Martin Splitt]

In general, as we're deprecating the project, you should look into alternative approaches to rendering on the web.