Add Igniter.Project.Config.remove_configuration

[ibarakaiev]

Is your feature request related to a problem? Please describe.
Currently, it is possible to configure keys, but not possible to remove existing configured keys.

Describe the solution you'd like
Config.remove_configuration or Config.remove_key or something like that.

Describe alternatives you've considered
It's currently possible to run Config.configure/6 and pass nil or [] or Keyword.remove_keyword_key/2 to clear the configuration, but it would be ideal if there's a dedicated function to remove configuration instead, which should also remove the entire config :key ... line as well if everything became empty.