Rewrite by eliminating the unavailable API

https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api

go.mod

@@ -3,10 +3,9 @@ module slack-dump
 go 1.16
 
 require (
-	github.com/cpuguy83/go-md2man/v2 v2.0.0 // indirect
+	github.com/cpuguy83/go-md2man/v2 v2.0.1 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
-	github.com/russross/blackfriday/v2 v2.1.0 // indirect
-	github.com/slack-go/slack v0.8.1
+	github.com/slack-go/slack v0.9.5
 	github.com/stretchr/testify v1.7.0 // indirect
 	github.com/urfave/cli/v2 v2.3.0
 )

go.sum

@@ -1,7 +1,7 @@
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
-github.com/cpuguy83/go-md2man/v2 v2.0.0 h1:EoUDS0afbrsXAZ9YQ9jdu/mZ2sXgT1/2yyNng4PGlyM=
-github.com/cpuguy83/go-md2man/v2 v2.0.0/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
+github.com/cpuguy83/go-md2man/v2 v2.0.1 h1:r/myEWzV9lfsM1tFLgDyu0atFtJ1fXn261LKYj/3DxU=
+github.com/cpuguy83/go-md2man/v2 v2.0.1/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -18,8 +18,8 @@ github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQD
 github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
 github.com/shurcooL/sanitized_anchor_name v1.0.0/go.mod h1:1NzhyTcUVG4SuEtjjoZeVRXNmyL/1OwPU0+IJeTBvfc=
-github.com/slack-go/slack v0.8.1 h1:NqGXuzni8Is3EJWmsuMuBiCCPbWOlBgTKPvdlwS3Huk=
-github.com/slack-go/slack v0.8.1/go.mod h1:FGqNzJBmxIsZURAxh2a8D21AnOVvvXZvGligs4npPUM=
+github.com/slack-go/slack v0.9.5 h1:j7uOUDowybWf9eSgZg/AbGx6J1OPJB6SE8Z5dNl6Mtw=
+github.com/slack-go/slack v0.9.5/go.mod h1:wWL//kk0ho+FcQXcBTmEafUI5dz4qz5f4mMk8oIkioQ=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=