Support for files with Graphviz(dot) syntax

[rwieckowski]

Generate preview for the entire file when Graphviz(dot) syntax selected

[munro]

747ed23
@rwieckowski hey I branched off your PR, let me know if this commit works for your workflow. What it does it 1) try selection 2) try surround snippet 3) try entire file. also made it so if there's a graphviz syntax error, it shows in a popup