Minor new features and maintenance update

CHANGELOG

• Dialer: add optional method NetDialTLSContext (#746) @lluiscampos
• Update README (#757) @garyburd
• Remove support for Go 1.8 (#755) @garyburd
• Improve protocol error messages (#754) @garyburd
• Update autobahn example (#753) @garyburd
• Fix broadcast benchmarks (#542) @FZambia
• Use context.Context in TLS handshake (#751) @garyburd
• Modify http Method String Literal to Variable (#728) @thak1411
• Update to match gofmt 1.17 (#750) @garyburd
• Document the allowed concurrency on Upgrader and Dialer (#636) @ghost
• improve echo example (#671) @hellflame
• build: use build matrix; drop Go <= 1.10 (#629) @elithrar
• Fix Docs w.r.t. setting subprotocols (#627) @elithrar
• Fix how the client checks for presence of Upgrade: websocket, Connection: upgrade (#604) @bluetech