Extend apis of low level serve functions to accept parser configuration

[cprussin]

Per @paluh in #129:

Maybe we can extend low level API of serve' and serveSecure' so they accept additional configuration which can be used in our final fromHTTPRequest for parsing HTTP.Request parts. Then we can keep our nice default high level API but also allow users to provide their own parsers so they can avoid additional per request parsing from our side which they are not interested in.

[citizengabe]

The middleware architecture could support this if the body was a concrete type instead of a typeclass as discussed in #115.