Parsing misses nested markdown.

[MichaelHatherly]

Elements such as List, Link, Bold, Italic don't contain parsed markdown, but rather just the original plain text.

julia> Markdown.parse("**`code`**").content
1-element Array{Content,1}:
 Paragraph(Content[Plain(""),Bold("`code`"),Plain("")])

julia> Markdown.parse("""
       * **bold**
       * *italic*
       * `code`
       """).content
1-element Array{Content,1}:
 List(Content[Plain("**bold**"),Plain("*italic*"),Plain("`code`")],true)

julia> Markdown.parse("[`code`](#code)").content
1-element Array{Content,1}:
 Paragraph(Content[Plain(""),Link("`code`","#code"),Plain("")])

[MikeInnes]

Fixed on master