Allow and/or automatically replace indented code blocks

[tlienart]

In Jekyll and elsewhere it seems that people like the 4-space or 1-tab indented stuff like

    this is some
    code

which would appear as such. At the moment JuDoc only takes blocks that are fenced with ``` as code blocks (and this should be encouraged anyway to be compatible with the code execution thing)

[tlienart]

actually this is going to be a lot easier than initially thought given that Julia's Markdown parser properly finds these things:

julia> a = """
blah `single` and

    1+1
    2+2

end
"""

julia> c = Markdown.parse(a).content
Markdown.Paragraph(Any["blah ", Markdown.Code("", "single"), " and"])
Markdown.Code("", "1+1\n2+2")                                        
Markdown.Paragraph(Any["end"])

julia> c[2].language = "julia";

julia> Markdown.html.(c)
3-element Array{String,1}:
 "<p>blah <code>single</code> and</p>"                              
 "<pre><code class=\"language-julia\">1&#43;1\n2&#43;2</code></pre>"
 "<p>end</p>"

So I think what remains to do is:

• allow the user to use a @def for the default-language to be applied on indented code blocks (which by default would be julia)
• in the md2html, update the pipeline so that Markdown.Code blocks are interpreted and appropriately modified