Option to generate a dot file representing the smir-json

[jberthold]

In this first version we don't draw the statements of blocks, just the block structure and caller/callee relations between blocks/items.

In terms of interface, the main program now has options --json and --dot which have to come first, and we default to --json if they are absent.

Output example: (main calls foo(0) calls bar(0))

The HS version had more color but was otherwise the same:

[jberthold]

At the moment this does not have tests...
We could add golden tests as for the smir.json output, but we'd have to normalise the output in some way, too, which seems to be overdone for the purpose.

[dkcumming]

This awesome for a first cut. Let's go with it as is and sometime down the track we can improve it. I'll approve now but I think we should add an issue for the statement printing, and an issue for testing the output. Also I think a line added to the README.md would be good. Even something as short as.

Experimental graph printing is available through the --dot flag