Add default severity level in config

[nchen63]

Need to provide a default severity level in the config file in case people want to set most rules to "warning" (we currently default to "error")

Questions to answer:

• does a new default severity just override the old default or does it wipe out all of the inherited custom levels?
  • if it doesn't wipe out inherited severity for each rule, could be annoying to have to override every rule individually
  • if it does wipe out everything, might be difficult to build a nuanced rule set
• should there be a rule level severity default?
  • this would allow base ruleset authors to provide flexibility
  • could add unnecessary complexity