Allow requests to pass through exec middleware

[andrewbrey]

Rather than always responding to the request with the result of the executed command, it can be desirable to simply run commands as a side-effect of handling a route in a given middleware chain. This enables use cases such as:

• regeneration of static assets just before serving them
• (re)starting proxy servers just prior to sending them a request
• alerting when certain routes are accessed
• etc

Note that the configuration name "pass_thru" was chosen to mirror the analogous configuration found in the file_server module: https://caddyserver.com/docs/caddyfile/directives/file_server#pass_thru

Implements #15

[andrewbrey]

@abiosoft I've updated this PR to include the changes you just merged in from #17 - sorry for any notification spam that was generated by GitHub, I tried to resolve the conflicting files in the UI, but that was more trouble than it was worth and had to re-fix from command line 😬