Add support for github task lists (aka checkboxes)  actually fixes dart-lang/markdown#268  add color swatch syntax support (dart-lang/markdown#428)

* change source of emojis to github api

* add github checkboxes and color syntax

* update change log

* update extension sets

* correct change log

* clean up, rename color_syntax.dart to color_swatch_syntax.dart

* remove unintentional emoji changes

* restore to master version

* restore to master version

* move ColorSwatchSyntax to gitHubWeb extension set

* add exports for new syntaxes

* made requested changes

* clean up so variables and comments as code review suggested