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/helpers.go

@@ -0,0 +1,164 @@
+// 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 parseLinkLabel(s *StateInline, start int, disableNested bool) int {
+	src := s.Src
+	labelEnd := -1
+	max := s.PosMax
+	oldPos := s.Pos
+
+	s.Pos = start + 1
+	level := 1
+	found := false
+
+	for s.Pos < max {
+		marker := src[s.Pos]
+
+		if marker == ']' {
+			level--
+			if level == 0 {
+				found = true
+				break
+			}
+		}
+
+		prevPos := s.Pos
+
+		s.Md.Inline.SkipToken(s)
+
+		if marker == '[' {
+			if prevPos == s.Pos-1 {
+				level++
+			} else if disableNested {
+				s.Pos = oldPos
+				return -1
+			}
+		}
+	}
+
+	if found {
+		labelEnd = s.Pos
+	}
+
+	s.Pos = oldPos
+
+	return labelEnd
+}
+
+func parseLinkDestination(s string, pos, max int) (url string, lines, endpos int, ok bool) {
+	start := pos
+	if pos < max && s[pos] == '<' {
+		pos++
+		for pos < max {
+			b := s[pos]
+			if b == '\n' || byteIsSpace(b) {
+				return
+			}
+			if b == '>' {
+				endpos = pos + 1
+				url = unescapeAll(s[start+1 : pos])
+				ok = true
+				return
+			}
+			if b == '\\' && pos+1 < max {
+				pos += 2
+				continue
+			}
+
+			pos++
+		}
+
+		return
+	}
+
+	level := 0
+	for pos < max {
+		b := s[pos]
+
+		if b == ' ' {
+			break
+		}
+
+		if b < 0x20 || b == 0x7f {
+			break
+		}
+
+		if b == '\\' && pos+1 < max {
+			pos += 2
+			continue
+		}
+
+		if b == '(' {
+			level++
+		}
+
+		if b == ')' {
+			if level == 0 {
+				break
+			}
+			level--
+		}
+
+		pos++
+	}
+
+	if start == pos {
+		return
+	}
+	if level != 0 {
+		return
+	}
+
+	url = unescapeAll(s[start:pos])
+	endpos = pos
+	ok = true
+
+	return
+}
+
+func parseLinkTitle(s string, pos, max int) (title string, nlines, endpos int, ok bool) {
+	lines := 0
+	start := pos
+
+	if pos >= max {
+		return
+	}
+
+	marker := s[pos]
+
+	if marker != '"' && marker != '\'' && marker != '(' {
+		return
+	}
+
+	pos++
+
+	if marker == '(' {
+		marker = ')'
+	}
+
+	for pos < max {
+		switch s[pos] {
+		case marker:
+			endpos = pos + 1
+			nlines = lines
+			title = unescapeAll(s[start+1 : pos])
+			ok = true
+			return
+		case '\n':
+			lines++
+		case '\\':
+			if pos+1 < max {
+				pos++
+				if s[pos] == '\n' {
+					lines++
+				}
+			}
+		}
+		pos++
+	}
+
+	return
+}