agg display current graph created with agn and age (see also ag-)

• agg show current graph in ascii art
• aggk show graph in key=value form
• aggi enter interactive mode for the current graph
• aggd print the current graph in GRAPHVIZ dot format
• aggv run graphviz + viewer (see 'e cmd.graph')
• agg* in r2 commands, to save in projects, etc