Split off Loaders

[clue]

The current classes in the Loader sub-namespace are pretty much useless and should be removed from this library. They haven't received any updates in years, lack any sort of documentation or tests and use a proprietary format.

Note that the CompleteGraph loader doesn't quite fit in there and will probably be moved to a separate namespace (see #116). Update: Postponed for now.

As a first step, I'd vote to split off the namespace into a separate composer package. Any input on naming is welcome. Perhaps something like graphp/plain?

Refs #120.