feat: Support modifying events within a callback function

[kattrali]

Goal

Support passing a function to Notify() to allow modification of the complete event, including the stacktrace, on a per-invocation basis.

bugsnag.Notify(err, func(event *bugsnag.Event) {
    event.Context = fmt.Printf("logger: %v", err)
})

Design

Check for a function argument to Notify(), aggregating any found into a list which is called once the error is constructed. Like other arguments to Notify(), subsequent arguments can override previous ones, which is why the callbacks are stored as a collection rather than overriding a single instance.

The StackFrame structure is now public to support checking against or modifying stack frame contents. There's an example in the changelog for a common case: trimming stacktraces for functions routed through a logger or common error delivery point.

Testing

• Tested manually that callbacks are executed and in order on multiple Go versions (1.8 and 1.15)
• Added new automated tests for common cases