Extensible flag parsers.

- use kiji-delegation to register flag parsers
 - @Flag annotation allows to specify a custom parser.
 - add default parsers for primitive object types (Integer, etc) that are nullable
 - parser for enums
 - adjusted the parsing of booleans (previously: --flag=yes would mean false)
 - ability to register parsers for subclasses (eg. Avro records)