add attempt_atom flag

[RoadRunnr]

add attempt_atom flag, which is compatible with

jsx:decode(Bin, [{label, attempt_atom}])

this is essentially #114 ported to master with a few small changes.

[RoadRunnr]

Note: I've also been thinking about adding to full variety of the jsx label options. That would result in some more extensive changes. Let me know if you prefer that over this simple change.