IPv6 remotes support (closes jpillora#197, thanks @shawnhwei)

* Support IPv6 remotes

* Adjust validation for IPv6 remotes

* IPv6 refactoring

* Further loosen remote part matching regex

fix Server.Start error (closes jpillora#195)

merge client stdio support (closes jpillora#166 thanks @BoleynSu!)

minor doc changes, test docker build

doc changes, fixed docker auto-build, moved licence to standalone file

merge client reverse+socks PR by @aus (closes jpillora#78)

- @aus's branch also includes PR for
custom headers, thanks @AkeemMcLennon  (closes jpillora#90)
- support client connections via socks
- add more architectures to the built releases
- remove vendor (go.sum will enforce correct deps)

upgrade deps, rename test/ to bench/, add goreleaser

add a the ability to add/delete users dynamically (jpillora#107)

* add a the ability to add/delete users dynamically

* update the AddUser function to support multiple authorized addrs