Revert graphviz back to v2.28.0
use config.h file and exclude -g for normal builds
revert layout arg; use layout graph attrib instead
camelCased viz (not a class); add fn + pkg exports
mv README.txt to README.md; updated with more docs
make C return a string since stdout works in node
reverted removal of cleanup code

Put in a call to gvFinalize() to mimic the execution of the dot command-line tool
rebuilt viz.js file