Back ticks fail to terminate a meta named fenced codeblock.

[grosenberg]

Added a custom block emitter to process blocks like

``` dot

digraph M1{ 
	node[shape=box width=1.1]
	dot[label="Graphviz\nDOT"]
	zestCode[label="Zest\ngraph"]
	zestVis[label="SWT\napp"]
	image[label="Image\nfile"]
	
	dot->image//[label=" Graphviz"]
	dot->zestCode[constraint=false color=black style=dashed label="            " dir=both]
	zestCode->zestVis//[label=" Zest"]	
}

~~~

Problem is that when the trailing delimiter of the code block is three back-ticks: ```, the list of lines delivered to the custom BlockEmitt#emitBlock includes every line from the first line of fenced code to the end of the document.

If the trailing delimiter is changed to ~~~, as show above, TxtMark behaves correctly.

If the custom block emitter is not used, the problem remains when using the trailing back-tick delimiter.

Suggests that something in this Dot content is messing with TxtMark. Just don't see what it is.