Add :GraphvizInteractive command

:GraphvizInteractive opens a dot viewer that immediately redraws the
image when the source file is saved. This is done with "dot -Txlib", the
same command that the vimdot script from the graphviz package uses.

Tested on Linux (ok) and Windows (not ok; graphviz isn't built with xlib
support).
Merge pull request #8 from bjornfor/graphviz-interactive

Add :GraphvizInteractive command