Add option to use certain commands outside of a ctfcli project

[MilyMilo]

• extract challenge resolving code from each command to a reusable method
• stop relying on config whenever it's not necessary (challenge new, format, lint)
• improve variable naming (explicit challenge_instance instead of shadowing the challenge kwarg)
• fix some typos

Closes #135

[MilyMilo]

I've made the challenge resolving methods static, so that they can be used outside of the challenge class (as utility functons).

I've also fixed 2 issues with plugins:

1. It was not possible to add a custom command, because fire always received the vanilla CTFCLI class, instead of the one that can be modified with plugins
2. __pycache__ inside the plugin directory would cause ctfcli to break