Add an option to define the graph ID in dot script

[clue]

When generating a Graph its name always is 'G' (in svg output it's the title sometimes showed as a web browser tooltip)

From graphp/graph#90. Originally reported by @clemens-tolboom.

[rhelms]

I'd like to do something to support this, and the workaround from the linked issue doesn't appear to work.

My suggestion would be to allow setting an id attribute on the Fhaculty\Graph\Graph, which would be used in the graph statement in Graphp\GraphViz\GraphViz.

$graph->setAttribute('graphviz.id', 'My Graph Title'); would result in graph 'My Graph Title' {, which should flow through to the SVG and resulting tooltip.

If not set, we could still default to G. And some extra testing can be done to make sure that an empty string works, even if '' needs to become ' '.

[rhelms]

I've just done some local code to see how it goes, and looking at the GraphViz doco, it might be better to set an attribute called graphviz.name to set the name of the graph.

I'll submit a pull request for this shortly.