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/markdown/fuzz.go

@@ -0,0 +1,9 @@
+//+build gofuzz
+
+package markdown
+
+func Fuzz(data []byte) int {
+	md := New(HTML(true))
+	md.Parse(data)
+	return 1
+}