Update generate/protocol.go to support newer measurement protocol HTML

The newer HTML seems to include some new hit-type parameters

https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters

This package is currently generated using an old HTML copy of the GA API docs, found here:

https://raw.githubusercontent.com/jpillora/go-ogle-analytics/master/generate/protocol-v1.html

It would be awesome if someone could update https://github.com/jpillora/go-ogle-analytics/blob/master/generate/protocol.go to support the new HTML.

protocol.go reads the HTML, copies relevant information into structs, then converts those structs into code, and finally, writes the code for each hit type into each of the type-*.go files found at the directory root.