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/parser_inline.go

@@ -0,0 +1,106 @@
+// 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 "unicode/utf8"
+
+type ParserInline struct {
+}
+
+type (
+	InlineRule      func(*StateInline, bool) bool
+	PostprocessRule func(*StateInline)
+)
+
+var (
+	inlineRules      []InlineRule
+	postprocessRules []PostprocessRule
+)
+
+func (i ParserInline) Parse(src string, md *Markdown, env *Environment) []Token {
+	if src == "" {
+		return nil
+	}
+
+	var s StateInline
+	s.Src = src
+	s.Md = md
+	s.Env = env
+	s.PosMax = len(src)
+	s.Tokens = s.bootstrap[:0]
+
+	i.Tokenize(&s)
+
+	for _, r := range postprocessRules {
+		r(&s)
+	}
+
+	return s.Tokens
+}
+
+func (ParserInline) Tokenize(s *StateInline) {
+	end := s.PosMax
+	src := s.Src
+	maxNesting := s.Md.MaxNesting
+	ok := false
+
+	for s.Pos < end {
+		if s.Level < maxNesting {
+			for _, rule := range inlineRules {
+				ok = rule(s, false)
+				if ok {
+					break
+				}
+			}
+		}
+
+		if ok {
+			if s.Pos >= end {
+				break
+			}
+			continue
+		}
+
+		r, size := utf8.DecodeRuneInString(src[s.Pos:])
+		s.Pending.WriteRune(r)
+		s.Pos += size
+	}
+
+	if s.Pending.Len() > 0 {
+		s.PushPending()
+	}
+}
+
+func (ParserInline) SkipToken(s *StateInline) {
+	pos := s.Pos
+	if s.Cache != nil {
+		if newPos, ok := s.Cache[pos]; ok {
+			s.Pos = newPos
+			return
+		}
+	} else {
+		s.Cache = make(map[int]int)
+	}
+
+	ok := false
+	if s.Level < s.Md.MaxNesting {
+		for _, r := range inlineRules {
+			s.Level++
+			ok = r(s, true)
+			s.Level--
+			if ok {
+				break
+			}
+		}
+	} else {
+		s.Pos = s.PosMax
+	}
+
+	if !ok {
+		_, size := utf8.DecodeRuneInString(s.Src[s.Pos:])
+		s.Pos += size
+	}
+	s.Cache[pos] = s.Pos
+}