Add TextBlock Widget

[Orasund]

The TextBlock Widget would add Markdown support. Its widget type should be an alias of pablohirafuji/elm-markdown's Block Type

type Block b i
    = BlankLine String
    | ThematicBreak
    | Heading String Int (List (Inline i))
    | CodeBlock CodeBlock String
    | Paragraph String (List (Inline i))
    | BlockQuote (List (Block b i))
    | List ListBlock (List (List (Block b i)))
    | PlainInlines (List (Inline i))
    | Custom b (List (Block b i))

(Checkout pablohirafuji/elm-markdown for more information)

The actual parsing would be done by pablohirafuji/elm-markdown.