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/heading.go

@@ -0,0 +1,59 @@
+// 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 ruleHeading(s *StateBlock, startLine, _ int, silent bool) bool {
+	pos := s.BMarks[startLine] + s.TShift[startLine]
+	max := s.EMarks[startLine]
+	src := s.Src
+
+	if s.SCount[startLine]-s.BlkIndent >= 4 {
+		return false
+	}
+
+	if pos >= max || src[pos] != '#' {
+		return false
+	}
+
+	level := 1
+	pos++
+	for pos < max && src[pos] == '#' && level <= 6 {
+		level++
+		pos++
+	}
+
+	if level > 6 || (pos < max && !byteIsSpace(src[pos])) {
+		return false
+	}
+
+	if silent {
+		return true
+	}
+
+	max = s.SkipSpacesBack(max, pos)
+	tmp := s.SkipBytesBack(max, '#', pos)
+	if tmp > pos && byteIsSpace(src[tmp-1]) {
+		max = tmp
+	}
+
+	s.Line = startLine + 1
+
+	s.PushOpeningToken(&HeadingOpen{
+		HLevel: level,
+		Map:    [2]int{startLine, s.Line},
+	})
+
+	if pos < max {
+		s.PushToken(&Inline{
+			Content: strings.TrimSpace(src[pos:max]),
+			Map:     [2]int{startLine, s.Line},
+		})
+	}
+	s.PushClosingToken(&HeadingClose{HLevel: level})
+
+	return true
+}