Allow graphviz-image to output image in different formats.

[dbrumann]

Tested locally (macOS) with svg, jpg & pdf and worked fine. When picking a format that is not valid, graphviz will conveniently tell us the supported formats:

 [ERROR]                                                                                                                
                                                                                                                        
         Output formatter graphviz-image threw an Exception:                                                            
                                                                                                                        
         Message: Unable to display output: An error occurred while creating the graph; GraphViz returned: Format:      
         "html" not recognized. Use one of: bmp canon cgimage cmap cmapx cmapx_np dot dot_json eps exr fig gd gd2 gif gv
         icns ico imap imap_np ismap jp2 jpe jpeg jpg json json0 mp pct pdf pic pict plain plain-ext png pov ps ps2 psd 
         sgi svg svgz tga tif tiff tk vdx vml vmlz vrml wbmp webp xdot xdot1.2 xdot1.4 xdot_json