Added data graph (aga) + partial support for diff graph (agd) output formats

[cyanpencil]

Now the command aga (which before was a dupe of ags) represents the data graph, which is basically a refs graph to data sections.

Also, I thought a lot about agd, but only graphviz and png formats can be supported because they are the only one that can show nodes background color. I'll return to agd in the future when I'll have implemented node backgrounds in ascii graphs.

Finally, I found out that agd was unfinished, and I tried to fix it myself... but after some hours I found out that's a rabbit hole much deeper than I expected, and I only fixed a small bug related to the fingerprinting of the bb nodes before diffing and added some more verbose error handling.

My doubts on agd are:

• Right now it does the diff between the function in the current offset and the function given in argument in the same binary... but does that really make sense? I think the right way should be what radiff2 does, that is diffing the same function between two different binaries.
• Found out that there is a command that does the diff of the function in the current offset and a function in a different file, that's called cgf, but it's broken and I think the name is wrong (Since it displays a graph it should be in the ag* family...)

[XVilka]

Please don't forget to add a couple regression tests for those, so it won't be broken after.