add entry-points

This refactors these modular aspects of coBib as entry-points, allowing them to be extended by plugins. Specifically, the following modules are refactored:

• cobib.commands
• cobib.importers
• cobib.parsers

This MR will also include a "dummy-plugin" showcasing a project structure, how to extend each of the above modules, and some ideas for testing them.

Since this will introduce some breaking changes, this will mark the start of v5.0 of coBib. The development of this MR is going to continue for a while, but before its conclusion I will make a separate MR to add some proper deprecations to the v4.5 and release that first. This allows this MR to continue with fully breaking changes without getting hung up on these details.

Implements #135 (closed)