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/lheading.go

@@ -0,0 +1,80 @@
+// 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 ruleLHeading(s *StateBlock, startLine, endLine int, silent bool) bool {
+	nextLine := startLine + 1
+
+	if s.SCount[startLine]-s.BlkIndent >= 4 {
+		return false
+	}
+
+	oldParentType := s.ParentType
+	s.ParentType = ptParagraph
+	src := s.Src
+
+	var pos int
+	var hLevel int
+outer:
+	for ; nextLine < endLine && !s.IsLineEmpty(nextLine); nextLine++ {
+		if s.SCount[nextLine]-s.BlkIndent > 3 {
+			continue
+		}
+
+		if s.SCount[nextLine] >= s.BlkIndent {
+			pos = s.BMarks[nextLine] + s.TShift[nextLine]
+			max := s.EMarks[nextLine]
+
+			if pos < max {
+				marker := src[pos]
+
+				if marker == '-' || marker == '=' {
+					pos = s.SkipBytes(pos, marker)
+					pos = s.SkipSpaces(pos)
+
+					if pos >= max {
+						hLevel = 1
+						if marker == '-' {
+							hLevel++
+						}
+						break
+					}
+				}
+			}
+		}
+
+		if s.SCount[nextLine] < 0 {
+			continue
+		}
+
+		for _, r := range paragraphTerminatedBy {
+			if r(s, nextLine, endLine, true) {
+				break outer
+			}
+		}
+	}
+
+	if hLevel == 0 {
+		return false
+	}
+
+	s.Line = nextLine + 1
+
+	s.PushOpeningToken(&HeadingOpen{
+		HLevel: hLevel,
+		Map:    [2]int{startLine, s.Line},
+	})
+	s.PushToken(&Inline{
+		Content: strings.TrimSpace(s.Lines(startLine, nextLine, s.BlkIndent, false)),
+		Map:     [2]int{startLine, s.Line - 1},
+	})
+	s.PushClosingToken(&HeadingClose{HLevel: hLevel})
+
+	s.ParentType = oldParentType
+
+	return true
+}