contrib/bst-graph: Add script to print graph in DOT format

This script leverages the recently added format strings
(`%{build-deps}`, `%{runtime-deps}`) to `bst show` to print a graph in
DOT format. This requires users to have the `graphviz` python package
installed.

Additionally, users can also render the graph using the `--format`
option if they have the `graphviz` command line tool installed.