[WIP] chi v4 + Go modules

[VojtechVitek]

Fixes #302

• chi v4 requires Go 1.10.3+ (or Go 1.9.7+)
• chi v4 is a Go module and thus breaks the import path; use github.com/go-chi/chi/v4 and github.com/go-chi/chi/v4/middleware

Upgrading your codebase from chi v3.x to chi v4

Example using GNU find (brew install find):

$ find . -type f -name '*.go' -not -path "./vendor/*" -exec sed -i 's+github.com/go-chi/chi+github.com/go-chi/chi/v4+g' {} \;

We recommend vendoring the dependencies via GO111MODULE=on go mod vendor && GO111MODULE=on go mod tidy.

If your repository still pulls in the old import path, you can figure out why by running GO111MODULE=on go mod why github.com/go-chi/chi.

[VojtechVitek]

This PR was rejected by the main chi author. He doesn't like the idea of changing the import paths because of Go modules support (ie. github.com/go-chi/chi/v4) and become a tech support for the confused chi users. So we're keeping it simple and chi stays on +incompatible flag for now.