Rewrite markdown rendering to blackfriday v2 and rewrite orgmode rendering to go-org (#8560)

* Rewrite markdown rendering to blackfriday v2.0

* Fix style

* Fix go mod with golang 1.13

* Fix blackfriday v2 import

* Inital orgmode renderer migration to go-org

* Vendor go-org dependency

* Ignore errors :/

* Update go-org to latest version

* Update test

* Fix go-org test

* Remove unneeded code

* Fix comments

* Fix markdown test

* Fix blackfriday regression rendering HTML block

vendor/github.com/russross/blackfriday/v2/.travis.yml

@@ -0,0 +1,17 @@
+sudo: false
+language: go
+go:
+  - "1.10.x"
+  - "1.11.x"
+  - tip
+matrix:
+  fast_finish: true
+  allow_failures:
+    - go: tip
+install:
+  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
+script:
+  - go get -t -v ./...
+  - diff -u <(echo -n) <(gofmt -d -s .)
+  - go tool vet .
+  - go test -v ./...