Eliminate panics (#459)

- Now, it is only(ideally) the config package that panics at startup if one or more options is misconfigured.
  The idea is that `ong` should return those errors upward rather than panic. If it has to panic, it should ideally do so only at one place; configuration.
- There are still some panics in `config/cert.go`. But those are only used when someone is using the `config.DevOpts`. We could remove them by using something like #277, #282

CHANGELOG.md

@@ -5,6 +5,7 @@ Most recent version is listed first.
 
 # v0.1.2
 - ong/middleware: Fix a number of CORS issues: https://github.com/komuw/ong/pull/442
+- ong/middleware: Eliminate panics: https://github.com/komuw/ong/pull/459
 
 # v0.1.1
 - ong/middleware: do not show hint: https://github.com/komuw/ong/pull/457