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/paragraph.go

@@ -0,0 +1,51 @@
+// 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"
+
+var paragraphTerminatedBy []BlockRule
+
+func ruleParagraph(s *StateBlock, startLine, _ int, _ bool) bool {
+	nextLine := startLine + 1
+	endLine := s.LineMax
+
+	oldParentType := s.ParentType
+	s.ParentType = ptParagraph
+
+outer:
+	for ; nextLine < endLine && !s.IsLineEmpty(nextLine); nextLine++ {
+		if s.SCount[nextLine]-s.BlkIndent > 3 {
+			continue
+		}
+
+		if s.SCount[nextLine] < 0 {
+			continue
+		}
+
+		for _, r := range paragraphTerminatedBy {
+			if r(s, nextLine, endLine, true) {
+				break outer
+			}
+		}
+	}
+
+	content := strings.TrimSpace(s.Lines(startLine, nextLine, s.BlkIndent, false))
+
+	s.Line = nextLine
+
+	s.PushOpeningToken(&ParagraphOpen{
+		Map: [2]int{startLine, s.Line},
+	})
+	s.PushToken(&Inline{
+		Content: content,
+		Map:     [2]int{startLine, s.Line},
+	})
+	s.PushClosingToken(&ParagraphClose{})
+
+	s.ParentType = oldParentType
+
+	return true
+}