Restrict component usage by configuration file

[thanethomson]

At present we can freely entries for components that potentially don't exist (i.e. allowing for spelling mistakes, or even the use of components in projects where components are generally not used).

This should be restricted by way of the configuration file, so that an error is produced if:

1. no components are defined and a user tries to add an entry for a component, or
2. the user tries to add an entry for a component not defined in the configuration file.