include update hooks for plugins/analyzers + extend yara signatures

[RootLUG]

This repository: https://github.com/Yara-Rules/rules looks like a very good candidate for including built-in yara rules, especially the packer and obfuscation detection rules.

As this is a third-party repo, an update mechanism should be in place to provide the latest signatures without manually checking for updates in the yara rules. This could be accomplished (ideally) by extending the aura update with update hooks that would allow installed plugins/analyzers to call their own update operations.