src - Split run function
src/ui - Check if modules are specified

At least one scan and one report module has to be specified
src - Add exceptions for config errors
src - Add two features to config options

* Raise exception if an error occurs while setting a config value e.g.
  type mismatch
* If the 'values' param of a choice option is a function than call it and
  use the returned value
src - Use a function to determine all rating mods
src - Renaming scanner to _scanner
src/ui - Print error msg if rating option is wrong