Command digraph

The digraph command performs queries over unlabelled directed graphs represented in text form. It is intended to integrate nicely with typical UNIX command pipelines.

Since directed graphs (import graphs, reference graphs, call graphs, etc) often arise during software tool development and debugging, this command is included in the go.tools repository.

TODO(adonovan): - support input files other than stdin - suport alternative formats (AT&T GraphViz, CSV, etc),

a comment syntax, etc.

- allow queries to nest, like Blaze query language.