Allow specifying preview host as option to preview command

[xortive]

Currently the preview host is hardcoded to example.com. We should allow people to configure it when running wrangler preview so they don't have to change it in the UI after the preview window opens.

[steveklabnik]

I wonder if we can't use the route for this...

[EverlastingBugstopper]

+1 on making this configurable

@steveklabnik this is something we ran into when implementing http events in our serverless framework. It's a little wonky because routes have wildcards - and there may be multiple endpoints (and multiple headers) you want to test with for a given worker. Defaulting to something that triggers the route would be good but I think we should also allow the user to pass in at least a url, if not also the method + headers.

[rochapablo]

any update?

[EverlastingBugstopper]

@rochapablo you can follow along with this pull request #1001 and also keep an eye out for an alpha release of wrangler dev.

[EverlastingBugstopper]

solved by #1001