Add an easy way to extend stack_trace_args

It's easy to accidentally stomp on the frame_filter that hides the
machinery of StackTrace::Auto from the generated stack trace, so we add
an overrideable `extra_stack_trace_args` method to the role. Then
consuming roles and classes can add to the argument list without having
to either subclass Devel::Stacktrace, copy/paste the default filter
code, or wrap/call a private method (`_build_stack_trace_args`).

I'm not sure the documentation is quite what it could be, and I'm in two
minds about whether to try and be clever if one of the keys in 
`extra_stack_trace_args` is `frame_filter` (though I lean towards trusting 
(handing enough rope to) anyone who wants to write their own `frame_filter`
and getting out of the way.)