Added verbose target which makes emscripten verbose
Fixed memory leak and cached Graphviz context

There is no need to create and destroy the context every time.
The returned pointer to char used to leak; now we save the last
result and free it before returning the next one.
Fixed agraphattr_init emscripten invalid cast warning
Merge branch 'master' of https://github.com/vmarkovtsev/viz.js into vmarkovtsev-master