Support all output formats for the refs graph (agr)

[cyanpencil]

Now agr supports every output format.

Other minor things:

• Adjusted aggj json output (sometimes there was a trailing comma)
• Removed e graph.layout = 1 from agr* (horizontal layout has broken edges... for now, better stick to the normal one)
• Fixed a bug when changing layout with = in the interactive view that would crash if the graph was a custom one (aggv or aggi)
• Small refactoring to make some function names more clear
• Running agr without argument now defaults to using core->offset rather than using offset 0x0 (which is much more intuitive imho)

[radare]

Use more descriptive commit messages next time. Not just in here