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/table_fsm.go

@@ -0,0 +1,108 @@
+// 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 isHeaderLine(s string) bool {
+	if s == "" {
+		return false
+	}
+
+	st := 0
+	n := 0
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		switch st {
+		case 0: // initial state
+			switch b {
+			case '|':
+				st = 1
+			case ':':
+				st = 2
+			case '-':
+				st = 3
+				n++
+			case ' ':
+				break
+			default:
+				return false
+			}
+
+		case 1: // |
+			switch b {
+			case ' ':
+				break
+			case ':':
+				st = 2
+			case '-':
+				st = 3
+				n++
+			default:
+				return false
+			}
+
+		case 2: // |:
+			switch b {
+			case ' ':
+				break
+			case '-':
+				st = 3
+				n++
+			default:
+				return false
+			}
+
+		case 3: // |:-
+			switch b {
+			case '-':
+				break
+			case ':':
+				st = 4
+			case '|':
+				st = 5
+			case ' ':
+				st = 6
+			default:
+				return false
+			}
+
+		case 4: // |:---:
+			switch b {
+			case ' ':
+				break
+			case '|':
+				st = 5
+			default:
+				return false
+			}
+
+		case 5: // |:---:|
+			switch b {
+			case ' ':
+				break
+			case ':':
+				st = 2
+			case '-':
+				st = 3
+				n++
+			default:
+				return false
+			}
+
+		case 6: // |:--- SPACE
+			switch b {
+			case ' ':
+				break
+			case ':':
+				st = 4
+			case '|':
+				st = 5
+			default:
+				return false
+			}
+		}
+	}
+
+	return n >= 1
+}