Add hook for pre- and post-parse

[stoically]

In some cases consumers want to operate on the token stream before it gets parsed by syn-rsx or after it was parsed (and before syn expects the token stream to end). To support such scenarios adding a callback function as additional configuration is needed.

It might also be nice to have an hook for attribute keys specifically, as that would allow modifiers on keys, such as key?=value, which is a syntax supported by yew, afaik.