Feature request: Mark stack frames as important

[lanzz]

When capturing a large stack trace that exceeds the frame allowance, Raven will indiscriminately truncate frames in the middle of the stack, dropping their locals and context source lines. On the other hand, it already supports explicitly hiding stack frames by annotating them with a __traceback_hide__ = True local variable. I would like to have a similar way to annotate stack frames as important to me, so that they're never trimmed down; what immediately comes to mind is to interpret an explicit __traceback_hide__ = False to that effect, so that no new magic local variables will be needed. slim_frame_data should consider such frames as off-limits and trim down other, unmarked frames instead.