Clarify maintenance of Config* components in MAINTAINERS.txt

Initial proposal - based on current/past contributions and a super-wild guess with regard to how "permanent" the current contributors (obviously there are MANY more!) are going to be.

Three sections, because the components are pretty independent:

- Configuration system: i.e., Drupal\Core\Config, the low-level config system itself, including stuff like the config import/export mechanism and includes/config.inc.

- Configurable entity system: i.e., Configurable* class stuff, the high-level configuration entity things, which are not actually part of the Configuration system itself, but play a key role for module APIs and the config import mechanism. This is also where potential stuff like the ctools-override/-revert handling would pile up.

- Configuration module: i.e., config.module, which will probably provide a user interface to configuration functionality.

I picked the most obvious contributors in order to get this issue/patch started - but I did NOT actually ask/verify anyone.

This is a long-term commitment, so this absolutely needs verification.

For example, I dunno whether @heyrocker actually wants to continue maintenance beyond the D8 initiative, nor are key architectural contributors like @beejeebus, @davidstrauss, @chx, and others contained (yet), since I wasn't sure about their long-term plans/intentions of involvement. Perfectly super-happy to adjust this patch! :)

The Configuration module is empty for now, as I'm not sure 1) what exact functionality config.module will actually provide in the end (considering that Configurables will probably be moved into Drupal\Core) and 2) who's going to work on the UI functionality (if any).