Allow to customize severity of Rules

[bkolb]

At the moment there is no way to way to customize the severity of a rule. There is a TODO already in the code for that, but I cannot find the associated issue.

[ahoppen]

Synced to Apple’s issue tracker as rdar://140346103

[allevato]

#626 is a related issue; it talks about a different idea (allowing format rules to be configured to turn off the changes they would make but still emit findings), but it also discusses the idea of leveraging that configuration to do things like change the severity.

I'm inclined to leave this issue open though since the severity thing is more of an aside in that issue.