Remove/rename internal properties for `Node` types

[mortenpi]

We have these internal fields like .nxt and .first_child, and we do some unnecessary get/setproperty stuff:

MarkdownAST.jl/src/node.jl

Lines 103 to 137 in 99e0f82

	function Base.getproperty(node::Node{T}, name::Symbol) where T
	if name === :element
	getfield(node, :t)
	elseif name === :children
	NodeChildren(node)
	elseif name === :next
	getfield(node, :nxt)
	elseif name === :previous
	getfield(node, :prv)
	elseif name === :parent
	getfield(node, :parent)
	elseif name === :meta
	getfield(node, :meta)
	else
	# TODO: error("type Node does not have property $(name)")
	@debug "Accessing private field $(name) of Node" stacktrace()
	getfield(node, name)
	end
	end
	function Base.setproperty!(node::Node, name::Symbol, x)
	if name === :element
	setfield!(node, :t, x)
	elseif name === :meta
	setfield!(node, :meta, x)
	elseif name in propertynames(node)
	# TODO: error("Unable to set property $(name) for Node")
	@debug "Setting private field :$(name) of Node" stacktrace()
	setfield!(node, name, x)
	else
	# TODO: error("type Node does not have property $(name)")
	@debug "Accessing private field :$(name) of Node" stacktrace()
	setfield!(node, name, x)
	end
	end

We should clean that up and make sure that you can only access documented fields. Internally, we can use getfield and setfield! where necessary. But let's do that in 0.2.0.

X-ref: #19.