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/newline.go

@@ -0,0 +1,46 @@
+// 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
+
+func ruleNewline(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	src := s.Src
+
+	if src[pos] != '\n' {
+		return false
+	}
+
+	pending := s.Pending.Bytes()
+	pmax := len(pending) - 1
+	max := s.PosMax
+
+	if !silent {
+		if pmax >= 0 && pending[pmax] == ' ' {
+			if pmax >= 1 && pending[pmax-1] == ' ' {
+				pmax -= 2
+				for pmax >= 0 && pending[pmax] == ' ' {
+					pmax--
+				}
+				s.Pending.Truncate(pmax + 1)
+				s.PushToken(&Hardbreak{})
+			} else {
+				s.Pending.Truncate(pmax)
+				s.PushToken(&Softbreak{})
+			}
+		} else {
+			s.PushToken(&Softbreak{})
+		}
+	}
+
+	pos++
+
+	for pos < max && byteIsSpace(src[pos]) {
+		pos++
+	}
+
+	s.Pos = pos
+
+	return true
+}