Support feature flags for features that don't exist yet

[TjerkNan]

Is your feature request related to a problem? Please describe.
When a new feature is not part of the code, setting the feature flag for this feature causes next to crash.

Describe the solution you'd like
Next ignores feature flags it doesn't recognise yet.

 Started next.
 ERROR! Config provider Config.Reader failed with:
 ** (ArgumentError) errors were found at the given arguments:
 * 1st argument: not an already existing atom
  :erlang.binary_to_existing_atom("onyx", :utf8)