Provide a way for plugins to add middlewares to the stack

[xsawyerx]

In issue #600 we discuss having a Plack::Builder object. Once we figure out the plugin implementation (#599), we should a way in the API to add stuff to that middleware stack.

[dostermeier]

+1

For a specific example, it would be great if I could register a component that wraps around all requests that can intercept exceptions / errors and do something in response.

Example:

I have a REST API and parts of it will throw validation errors. I would like to have one place that says:

If the exception generated is of type ...Validation, then set status code to 400 and error = some message.