bithumb: Add websocket support (#745)

* bithumb: Add websocket support (WIP)

* bithumb: cont impl.

* bithumb: finish, issues with orderbook needs review

* linter issues

* bithumb: move to separate file

* bithumb: change to pointer for book

* bithumb: Add tests

* bithumb: Address nits

* websocket: Export subscription error and wrap returns

* bithumb: cleanup/ fix tests

* gctrpc/bithumb: fix misspelling

* bithumb: gofmt

* readme: update support table, regen doc

* tradesReadme: update

* readme: update template

gctrpc/rpc.swagger.json

@@ -1285,7 +1285,7 @@
           "200": {
             "description": "A successful response.",
             "schema": {
-              "$ref": "#/definitions/gctrpcGetExchangeOTPReponse"
+              "$ref": "#/definitions/gctrpcGetExchangeOTPResponse"
             }
           },
           "default": {
@@ -3793,7 +3793,7 @@
         }
       }
     },
-    "gctrpcGetExchangeOTPReponse": {
+    "gctrpcGetExchangeOTPResponse": {
       "type": "object",
       "properties": {
         "otpCode": {