Add ability to pass a custom allowed hosts function

[franklinexpress]

This allows us to pass a custom AllowedHosts func that returns a list of such hosts and overrides the AllowedHosts list

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Thanks for the PR, really like this idea! One comment on the code, and can you also update the README file with the new option?

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Looking good, just a couple suggestions!

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Suggested change

	if s.opt.AllowedHostsFunc != nil && !s.opt.IsDevelopment {
	s.opt.AllowedHosts = append(s.opt.AllowedHosts, s.opt.AllowedHostsFunc()...)
	}
	combinedAllowedHosts = s.opt.AllowedHosts
	if s.opt.AllowedHostsFunc != nil {
	combinedAllowedHosts = append(combinedAllowedHosts, s.opt.AllowedHostsFunc()...)
	}

This function is called on every request, so I think the current implementation will just append the function results every time. I'm thinking we can create a temporary variable and combine the static and dynamic lists instead.

[franklinexpress]

Hm, I actually need it to be called on every request. In my case, the list changes.

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My suggested change will still call the function on every request. The way the code is written now will append the results of the function call into s.opt.AllowedHosts on every request... so that array will grow and grow and grow! Instead of appending to s.opt.AllowedHosts, we can create a new variable that combines both the static list and the function call results. Then on lines 302 and 312 we can update the references from s.opt.AllowedHosts to the new variable we created (I called it combinedAllowedHosts in my suggestion above)

[franklinexpress]

Oh, of course, I see. Looked into the source more closely, thanks

[franklinexpress]

Am actually wondering if we can modify this to pass in the request to the function, what do you think? Or have another functionisHostAllowed(host string)

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I like the idea of passing the request in! I'll try to prototype that soon and see how it fits in.

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Thanks for all your work on this!

[franklinexpress]

Thanks for all your work on this!

Thanks for the feedback