Add support for markdown to HTML conversion (matrix). Closes #663 (#670)

This uses our own gomatrix lib with the SendHTML function which
adds HTML to formatted_body in matrix.
golang-commonmark is used to convert markdown into valid HTML.

vendor/gitlab.com/golang-commonmark/puny/.gitlab-ci.yml

@@ -0,0 +1,16 @@
+image: golang:1.11
+
+stages:
+  - build
+  - test
+
+build:
+  stage: build
+  script:
+    - go build ./...
+
+test:
+  stage: test
+  script:
+    - test -z "$(gofmt -l . | tee /dev/stderr)"
+    - go test ./...