split challenge.py into generic properties

[vellvoid]

core/challenge.py has become the designated place for most of ctfcli's most-important functionality (managing challenges). Adding a new property to the challenge requires touching multiple places and further stuffing of that file - the architecture is just not scalable.

This PR introduces a concept of properties - each challenge property like flags, topics, files, name etc. is handled within its own class which knows how to install, sync, compare, pull that specific property. The challenge class just calls hooks for all known properties.

Adding a new property boils down to creating a new class (or using a generic one for simple properties like just a string), and registering it with the properties registry.

[vellvoid]

this builds on #213 to avoid conflicts