* cmd/dlv: fall back to go env GOPATH when GOPATH envvar is unset.
Since Go 1.8, GOPATH environment variable has a default value if the
environment variable is unset so not setting GOPATH is perfectly ok.
Depend on `go env` command to query Go related environment variable.
* build: update references to moved packages
* documentation: copied old documentation from wiki
* command: better online documentation
Help without arguments prints just a short summary for each command,
help followed by a command prints the command's syntax and a longer
explanation.
* documentation: automatically generate Documentation/cli/README.md