Feature: add ssr support (#805)

* Refactor ssr stream cipher to expose iv and key

References:
https://github.com/Dreamacro/go-shadowsocks2
https://github.com/sh4d0wfiend/go-shadowsocksr2

* Implement ssr obfs

Reference:
https://github.com/mzz2017/shadowsocksR

* Implement ssr protocol

References:
https://github.com/mzz2017/shadowsocksR
https://github.com/shadowsocksRb/shadowsocksr-libev
https://github.com/shadowsocksr-backup/shadowsocksr

component/ssr/obfs/plain.go

@@ -0,0 +1,25 @@
+package obfs
+
+type plain struct{}
+
+func init() {
+	register("plain", newPlain)
+}
+
+func newPlain(b *Base) Obfs {
+	return &plain{}
+}
+
+func (p *plain) initForConn() Obfs { return &plain{} }
+
+func (p *plain) GetObfsOverhead() int {
+	return 0
+}
+
+func (p *plain) Encode(b []byte) ([]byte, error) {
+	return b, nil
+}
+
+func (p *plain) Decode(b []byte) ([]byte, bool, error) {
+	return b, false, nil
+}