Zerodoc

Added support for zerodoc format (https://github.com/odkq/zerodoc/ and https://pypi.python.org/pypi/zerodoc)

Full support would involve installing asciitosvg, ditaa & aafigure & tikz, but without diagrams support (the included file in test/markups/README.zd do not have diagrams) only `pip install zerodoc` is neccesary