Introduce Codenarc(Extension)#config

- based on new TextResource abstraction
- set to replace #configFile
Update open questions in design spec
Make some direct use of new way to configure findbugs filters in integration test
Introduce Pmd(Extension)#rulesetConfig

- new way to configure custom rule set based on TextResource abstraction
- may add support for multiple rule sets in the future, in order to match capabilities of ruleSetFiles property