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/fence.go

@@ -0,0 +1,102 @@
+// Copyright 2015 The Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package markdown
+
+import "strings"
+
+func ruleFence(s *StateBlock, startLine, endLine int, silent bool) bool {
+	haveEndMarker := false
+	pos := s.BMarks[startLine] + s.TShift[startLine]
+	max := s.EMarks[startLine]
+
+	if s.SCount[startLine]-s.BlkIndent >= 4 {
+		return false
+	}
+	if pos+3 > max {
+		return false
+	}
+
+	src := s.Src
+
+	marker := src[pos]
+	if marker != '~' && marker != '`' {
+		return false
+	}
+
+	mem := pos
+	pos = s.SkipBytes(pos, marker)
+
+	len := pos - mem
+
+	if len < 3 {
+		return false
+	}
+
+	params := strings.TrimSpace(src[pos:max])
+
+	if strings.IndexByte(params, marker) >= 0 {
+		return false
+	}
+
+	if silent {
+		return true
+	}
+
+	nextLine := startLine
+
+	for {
+		nextLine++
+		if nextLine >= endLine {
+			break
+		}
+
+		mem = s.BMarks[nextLine] + s.TShift[nextLine]
+		pos = mem
+		max = s.EMarks[nextLine]
+
+		if pos < max && s.SCount[nextLine] < s.BlkIndent {
+			break
+		}
+
+		if pos >= max || src[pos] != marker {
+			continue
+		}
+
+		if s.SCount[nextLine]-s.BlkIndent >= 4 {
+			continue
+		}
+
+		pos = s.SkipBytes(pos, marker)
+
+		if pos-mem < len {
+			continue
+		}
+
+		pos = s.SkipSpaces(pos)
+
+		if pos < max {
+			continue
+		}
+
+		haveEndMarker = true
+
+		break
+	}
+
+	len = s.SCount[startLine]
+
+	s.Line = nextLine
+	if haveEndMarker {
+		s.Line++
+	}
+
+	s.PushToken(&Fence{
+		Params:  params,
+		Content: s.Lines(startLine+1, nextLine, len, true),
+		Map:     [2]int{startLine, s.Line},
+	})
+
+	return true
+}