Update markdown parsing library to github.com/gomarkdown/markdown (#944)

vendor/modules.txt

@@ -51,6 +51,11 @@ github.com/go-telegram-bot-api/telegram-bot-api
 # github.com/golang/protobuf v1.3.1
 github.com/golang/protobuf/proto
 github.com/golang/protobuf/protoc-gen-go/descriptor
+# github.com/gomarkdown/markdown v0.0.0-20190912180731-281270bc6d83
+github.com/gomarkdown/markdown
+github.com/gomarkdown/markdown/ast
+github.com/gomarkdown/markdown/html
+github.com/gomarkdown/markdown/parser
 # github.com/google/gops v0.3.6
 github.com/google/gops/agent
 github.com/google/gops/internal
@@ -189,16 +194,6 @@ github.com/valyala/bytebufferpool
 github.com/valyala/fasttemplate
 # github.com/zfjagann/golang-ring v0.0.0-20190304061218-d34796e0a6c2
 github.com/zfjagann/golang-ring
-# gitlab.com/golang-commonmark/html v0.0.0-20180917080848-cfaf75183c4a
-gitlab.com/golang-commonmark/html
-# gitlab.com/golang-commonmark/linkify v0.0.0-20191026162114-a0c2df6c8f82
-gitlab.com/golang-commonmark/linkify
-# gitlab.com/golang-commonmark/markdown v0.0.0-20181102083822-772775880e1f
-gitlab.com/golang-commonmark/markdown
-# gitlab.com/golang-commonmark/mdurl v0.0.0-20180912090424-e5bce34c34f2
-gitlab.com/golang-commonmark/mdurl
-# gitlab.com/golang-commonmark/puny v0.0.0-20180912090636-2cd490539afe
-gitlab.com/golang-commonmark/puny
 # go.uber.org/atomic v1.4.0
 go.uber.org/atomic
 # go.uber.org/multierr v1.1.0
@@ -250,7 +245,6 @@ golang.org/x/text/secure/bidirule
 golang.org/x/text/transform
 golang.org/x/text/unicode/bidi
 golang.org/x/text/unicode/norm
-golang.org/x/text/unicode/rangetable
 # gopkg.in/natefinch/lumberjack.v2 v2.0.0
 gopkg.in/natefinch/lumberjack.v2
 # gopkg.in/yaml.v2 v2.2.2